The bug

When an SSE response ended before its JSON-RPC response arrived, HttpTransport discarded the stream. It did not retain the SSE id or retry fields, so the pending request waited for its timeout instead of resuming from the last event.

The fix

• Track the latest SSE event ID and the server-provided retry interval.
• Resume interrupted streams with a GET carrying Accept: text/event-stream, Mcp-Session-Id, and Last-Event-ID.
• By default, use 1/2/4/8/10 second fallback backoff and stop after five consecutive failed attempts. A server retry: value takes precedence.
• Stop reconnecting after a final response, timeout or cancellation, explicit close, or when the stream has no resumable event ID.
• Add the sse-retry conformance path and remove it from the expected-failure baseline.

Tests

• vendor/bin/phpunit — 1693 tests, 4416 assertions, 7 skipped
• vendor/bin/phpstan analyse --memory-limit=-1
• vendor/bin/php-cs-fixer fix --dry-run --diff --config=.php-cs-fixer.dist.php
• make docs
• composer validate --strict
• sse-retry conformance — 3/3

Fixes #327.