[moonyue-w]

Change

Fix four Java SDK transport behaviors:

• Repeated default headers now use the last value regardless of header-name casing; per-request overrides still take precedence.
• Realtime handshake and connection failures retain their underlying transport exception in RealtimeException.getCause().
• Resumable SSE subscriptions honor valid Retry-After and retry-after-ms after the per-handshake retry budget is exhausted. Cancellation and total deadlines interrupt these waits; the existing exponential backoff remains the fallback.
• Closing an active event subscription records cancellation before releasing the reader, preventing a race that previously reported successful completion.

Update the changelog, streaming guidance and stale Identity SAT / SSE / Realtime documentation. These changes are Java-specific; Go, Python and TypeScript are unaffected. No version bump or caller migration is required.

Validation

• SDK build, tests and Javadoc pass.
• Independent examples consume the installed JAR and pass.
• Java 8 package checks pass; changed runtime behavior is checked on Java 8 and Java 17.
• Documentation and changelog reflect public behavior changes.
• make quality passes on JDK 17 (including 38 maintenance-script tests and local-link checks).
• Version files are unchanged.

Executed make test on both Java 8 and Java 17: each passed 545 SDK tests and 62 packaged-consumer tests, with no failures, errors or skips. This also builds Javadoc and checks package contents and Java 8 bytecode. Re-ran make quality after the documentation updates.

Regression coverage includes mixed-case headers, synchronous/asynchronous handshake failures, retry delays after zero and nonzero retry budgets, cancellation/deadline interruption during server-directed waits, and deterministic reader/close interleaving. Local loopback probes on both runtimes confirmed a 3-second server delay was respected and all 2,000 explicit cancellations reported cancellation.

No live CN/Global calls, AoneCI runs or Maven publication were performed for this fix. Java 11/21/25 validation is delegated to the GitHub CI matrix. These offline results do not replace production acceptance or human QA approval.