feat!: Split LdAiCompletionConfig from LdAiCompletionConfigDefault

[jsonbailey]

BEGIN_COMMIT_OVERRIDE
feat!: Split unified LdAiConfig into LdAiCompletionConfig (SDK output) and LdAiCompletionConfigDefault (user input) — Builder, New(), and Disabled move to the Default type; introduce abstract LdAiConfig and LdAiConfigDefault base types for future agent/judge modes
feat!: Change CompletionConfig to return LdAiCompletionConfig instead of ILdAiConfigTracker — obtain a tracker via config.CreateTracker()
feat!: Default LdAiCompletionConfigDefault.Enabled to true per AISDK spec (was false on the LdAiConfig builder in 0.9.x)
feat!: Remove ILdAiConfigTracker.Config property — read config fields from the LdAiCompletionConfig the caller already holds
feat!: Make LdAiConfigTracker SDK-constructed only — the public constructor is removed; obtain trackers via config.CreateTracker() or ILdAiClient.CreateTracker(resumptionToken, context)
feat!: Convert LdAiCompletionConfig and LdAiCompletionConfigDefault from records to classes — equality is reference-based instead of structural
feat!: Remove LaunchDarkly.Sdk.Server.Ai.DataModel namespace — delete unused AiConfig, Meta, Model, Provider, and Message JSON DTO classes
feat: Tolerant LdValue parsing — missing or wrong-typed fields degrade to typed defaults instead of discarding the whole config
feat: Per-message interpolation fallback — a malformed Mustache template keeps raw content for that message rather than discarding the entire config
feat: Mode-mismatch detection — log a warning and return the caller's default when the flag's _ldMeta.mode does not match the requested mode (per sdk-specs#229)
feat: Non-object variation handling — log an error and return the caller's default when the variation result is not an object
END_COMMIT_OVERRIDE

Summary

This change splits the unified LdAiConfig from 0.9.x into two public sealed types so that the data a user supplies as a fallback is clearly separated from the data the SDK returns:

• LdAiCompletionConfig is what LdAiClient.CompletionConfig returns. It is constructed only by the SDK, exposes the evaluated config data, and provides a CreateTracker() method that always returns a non-null tracker. There is no public constructor, no public Builder, and no Disabled factory.
• LdAiCompletionConfigDefault is the user-input type passed as the defaultValue: parameter. It carries the prompt/model/provider data but has no tracker. Construction uses the public Builder, New(), and Disabled factory.

Tracker construction is driven by a required factory wired up at evaluation time inside LdAiClient. LdAiCompletionConfig.CreateTracker() delegates to that factory, so callers no longer have to deal with a nullable tracker -- even on fallback paths (parse failure, interpolation failure).

Type shape (Option B: parallel hierarchies)

Two abstract bases factor out the shared structure, but the Default and Live types do not inherit from each other:

LdAiConfigDefault (abstract)         LdAiConfig (abstract)
  Model, Provider, bool? Enabled       Key, Model, Provider, Enabled,
                                       VariationKey, Version,
                                       CreateTracker()
        |                                       |
LdAiCompletionConfigDefault         LdAiCompletionConfig
  + Messages, Builder, New, Disabled  + Messages, CreateTracker

Users should reference the concrete sealed types (LdAiCompletionConfig, LdAiCompletionConfigDefault). Shared payload types (Message, Role, ModelConfig, ProviderConfig) live under static LdAiConfigTypes in the Config namespace (see #284).

This matches the cross-SDK pattern (parallel hierarchies in python and ruby; live-extends-omit-default in js-core). Future LdAiAgentConfig / LdAiJudgeConfig slot in symmetrically -- each Default extends LdAiConfigDefault, each Live extends LdAiConfig.

Why this lands separately from #249

PR #249 contains concurrency fixes for the AI SDK. It rebased on top of this once this merged so that both reviews stay focused.

Migration

Before (0.9.x):

var defaultConfig = LdAiConfig.New().Enable().AddMessage("Hi").Build();
ILdAiConfigTracker tracker = client.Config("flag-key", ctx, defaultConfig);
LdAiConfig cfg = tracker.Config;

After (0.10.0):

var defaultConfig = LdAiCompletionConfigDefault.New().Enable().AddMessage("Hi").Build();
LdAiCompletionConfig cfg = client.CompletionConfig("flag-key", ctx, defaultConfig);
var tracker = cfg.CreateTracker();

The obsolete Config(...) method is preserved with the same [Obsolete] attribute; only its parameter and return types are updated to match.

Test plan

• dotnet build succeeds across netstandard2.0, net462, net8.0
• dotnet test --framework net8.0 passes (53/53)
• New test: reflection-based assertion that LdAiCompletionConfig has no public constructors (locks in the SDK-only-construction invariant)
• New tests: CreateTracker() returns a working tracker on both the parse-failure and interpolation-failure fallback paths
• Reviewer confirms migration shape matches the cross-SDK pattern

---

Note

High Risk
Large breaking public API surface (types, return shapes, tracker construction) plus new evaluation/fallback semantics that integrators must migrate to; behavior changes on parse/interpolation errors affect production prompts and telemetry wiring.

Overview
Breaking API redesign for the server AI SDK: unified LdAiConfig is replaced by LdAiCompletionConfig (SDK evaluation result) and LdAiCompletionConfigDefault (caller fallback with New/Builder/Disabled). CompletionConfig now returns the config object; usage tracking is config.CreateTracker() instead of receiving a tracker directly. ILdAiConfigTracker.Config is removed.

Evaluation logic moves into internal ConfigFactory, which parses flag JSON as LdValue (the DataModel JSON DTOs are deleted). LdAiClient is slimmed down to JsonVariation + factory build. Fallback behavior is more tolerant: wrong type or _ldMeta.mode mismatch uses the caller’s default (with logging); bad Mustache templates fail per message instead of dropping the whole config. Shared shapes live on LdAiConfigBase / LdAiConfigDefaultBase with top-level Message, ModelConfig, ProviderConfig, Role. Default builder Enabled defaults to true; main config types are classes (reference equality). LdAiConfigTracker is internal and keyed off config.Key.

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[mattrmc1]

A super minor nit but looks great. Pulled down and tested locally 👍

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