Strictly check variant types in Flag Evaluations

[typotter]

What Does This Do

Adds strict type checking to DDEvaluator so that requesting a flag evaluation with a type that doesn't match the flag's declared variationType returns TYPE_MISMATCH instead of silently coercing the value.

Also distinguishes variant value parse errors (broken config) from request type mismatches:

• Request type vs flag variationType → TYPE_MISMATCH (e.g., STRING flag evaluated as Boolean)
• Variant value vs declared type → PARSE_ERROR (e.g., INTEGER flag with string variant value)

Motivation

Aligns the Java SDK with Python (libdatadog) and Go (dd-trace-go#4590) type checking behavior. Previously, Java's mapValue() silently coerced types — Boolean.valueOf("on-value") returned false instead of erroring, and parseDouble("123").intValue() returned 123 instead of reporting a type mismatch.

Cross-tracer consistency is required for the FFE eval metrics system tests to pass (solves 2 of the 6 remaining test failures).

Type compatibility mapping:

variationType	Valid Java type
BOOLEAN	Boolean.class
STRING	String.class
INTEGER	Integer.class
NUMERIC	Double.class
JSON	Value.class

References:

• Go SDK consistency PR: fix(openfeature): improve FFE eval metrics cross-tracer consistency dd-trace-go#4590
• Python type checking: libdatadog flag_type.rs bitwise type check

Additional Notes

Contributor Checklist

• Format the title according to the contribution guidelines
• Assign the type: and (comp: or inst:) labels
• Avoid using close, fix, or any linking keywords when referencing an issue

Jira ticket: FFL-1972

[PerfectSlayer]

Hey @typotter 👋 Can you make sure the PR title follows the title format contribution guideline?
As the PR title goes straight into the release log for customers, please reword them accordingly :) Thanks!

[typotter]

Hey @typotter 👋 Can you make sure the PR title follows the title format contribution guideline? As the PR title goes straight into the release log for customers, please reword them accordingly :) Thanks!

done, thanks!