fix(server): derive standalone SSE streamId per-session to prevent EventStore collisions

[leehpham]

Fixes #1004. WebStandardStreamableHTTPServerTransport hardcoded _standaloneSseStreamId to the literal '_GET_stream', so when multiple transport instances shared an EventStore (e.g. Redis-backed for horizontal scaling), every session's GET-SSE notifications were stored under the same streamId - breaking per-session replay isolation.

This converts the field to a getter returning `_GET_stream_${sessionId}` once a session is initialized, preserving the original '_GET_stream' literal in stateless mode for backward compatibility. The _streamMapping is per-instance so its keying is unaffected.

Adds a regression test that runs two transports with distinct sessions against one shared EventStore and asserts the recorded streamIds are distinct, plus a stateless-mode test pinning the literal fallback.

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