refactor: add SSE Builder::on_response hook

[beekld]

Summary

Adds an on_response hook to sse::Builder, invoked once per (re)connect after the HTTP response headers have been received, before any branching on status. Fires for every response (any status, including redirects and errors); does not fire if the connection failed before any response (TCP error, read timeout).

Prep for upcoming FDv2 work that needs to inspect the X-LD-FD-Fallback response header.

Design notes

cURL backend accumulates header lines into a per-transfer http::response_header<> on RequestContext, because HeaderCallback is invoked once per CRLF-terminated line. The hook fires on the empty terminator and is posted to the client strand.

Test plan

• gtest_launchdarkly-sse-client — green under both Foxy and cURL backends
• New ClientTest.OnResponseHookFiresWithCustomHeader

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Note

Medium Risk
Touches reconnect/header parsing on both Foxy and cURL SSE paths; cURL header assembly is new logic though covered by existing and new client tests.

Overview
Adds sse::Builder::on_response, a callback that runs once per connect/reconnect right after response headers are parsed and before status-based handling (success, redirect, errors). It receives a http::response_header<> so callers can read headers such as X-LD-FD-Fallback without waiting for the SSE body.

Foxy backend invokes the hook synchronously on the client executor when header parsing completes. cURL backend rebuilds the prior Content-Type-only header sniffing into full line-by-line header assembly (status line, OWS trimming, duplicate headers via insert(), redirect chains), emits the hook on the blank line after headers, and posts it to the same executor as other callbacks. Header accumulator state is reset on each new transfer.

Includes ClientTest.OnResponseHookFiresWithCustomHeader for status and custom header visibility.

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Cursor Bugbot has reviewed your changes using default effort and found 1 potential issue.

There are 2 total unresolved issues (including 1 from previous review).

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[kinyoklion]

This one could still throw for invalid header names.

[beekld]

Fixed.