Add generic server-sent events support to Azure Core

[Xiaofei Cao (XiaofeiCao)]

Summary

Adds minimal, single-response server-sent events support to azure-core, based on #50081.

• Adds ServerSentEvent<T>, ServerSentEventListener<T>, and ServerSentEventStreams for synchronous listener and asynchronous Flux consumption.
• Incrementally parses text/event-stream responses and supports optional service-defined terminal-event predicates.
• Preserves live response bodies through REST proxy decoding and returns a closeable Response<BinaryData> implementation for streaming responses.
• Prevents HttpLoggingPolicy from buffering SSE response bodies.
• Closes the physical response on completion, failure, interruption, terminal events, or Reactor cancellation.
• Bounds incomplete events to 16 MiB and uses a single data buffer to prevent unbounded heap growth.

Scope

This PR intentionally handles one established HTTP response. It does not reconnect, replay requests, or send Last-Event-Id. Search client APIs, generated models, and emitter work remain separate.

Contract

Response overloads require a closeable HTTP 200 or 204 response. Non-204 responses require exactly one text/event-stream content type and reject an explicit non-UTF-8 charset. Existing overloads complete on EOF or HTTP 204; predicate overloads require and inclusively deliver a matching terminal event.

Validation

• Full azure-core test suite: 11,881 tests, 0 failures, 3 skipped
• Checkstyle: 0 violations
• SpotBugs: 0 findings

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