feat(binding-llm): SSE content decoder for text/event-stream

[jfallows]

Description

Implements LlmContentDecoderSpi for text/event-stream, decoding blank-line-delimited SSE framing (data:/event:/id: fields, comment lines, CRLF/CR/LF line endings) into event DATA + FLUSH frames, per the WHATWG SSE parsing algorithm. Framing decode only, independent of any dialect-specific (OpenAI, Anthropic, etc.) payload interpretation — feeds both same-dialect (framing-only) and cross-dialect (framing + payload transform) routes.

• LlmSseContentDecoder (io.aklivity.zilla.runtime.binding.llm.internal.decode) — decodes one stream's SSE framing, reporting data() for accumulated data: field bytes (joined with \n across multiple data: lines in one event) and flush(event, id, ...) on each blank-line dispatch. Comment lines (:...) and unrecognized fields (e.g. retry:) are ignored.
• LlmSseContentDecoderFactorySpi — registers the decoder for content-type text/event-stream, wired via META-INF/services and a provides clause added to module-info.java (the uses declaration already existed from feat(binding-llm): LlmContentDecoderSpi internal SPI, keyed by content-type #2553).
• LlmSseContentDecoderTest / LlmSseContentDecoderFactorySpiTest — 19 new unit tests covering multi-line data joining, event-name reset semantics, last-event-id persistence across dispatches (and NUL-byte rejection / buffer growth for it), comment/unrecognized-field handling, and the CR/CRLF line-ending edge cases including the ambiguous trailing-CR-at-buffer-edge case (deliberately left unconsumed since it may still turn out to be part of a CRLF pair once more bytes arrive).

Representational-slack note (per the issue's open question): this decoder normalizes field ordering and does not preserve field-value whitespace beyond the single optional space after the colon that the SSE spec itself strips. Re-encoding from the decoded data/flush frames alone cannot byte-exactly reproduce arbitrary source framing — a later round-trip test should assert semantic equality, not assertEquals(bytes).

Verified ./mvnw clean install -pl incubator/binding-llm.conf,incubator/binding-llm passes end-to-end: checkstyle, license headers, and all 27 unit tests pass with full jacoco coverage.

This branch is built on top of claude/issue-2478-76f5ry (#2553, the LlmContentDecoderSpi internal SPI), so this diff includes #2553's commits until that merges to develop — at which point this PR's diff will shrink to just this SSE decoder commit.

Fixes #2479

🤖 Generated with Claude Code

https://claude.ai/code/session_013kK9z4ywuPJe33jm6W1fik

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Generated by Claude Code

[jfallows]

Rebased onto the latest claude/issue-2478-76f5ry (#2553). That branch's refactor(binding-llm): collapse decoder/encoder SPIs into LlmContentCodecSpi commit already pulled this PR's LlmSseContentDecoder forward verbatim (widened to public) and re-registered it under the new LlmSseContentCodecSpi, replacing the LlmContentDecoderSpi/LlmSseContentDecoderFactorySpi abstraction this PR introduced.

After resolving the rebase conflicts (keeping the upstream codec-based registration and dropping the now-dead LlmSseContentDecoderFactorySpi + its test, which implemented the deleted LlmContentDecoderSpi), this branch has zero unique commits left relative to #2553 — issue #2479's SSE decoder is fully covered there.

Leaving this open with a zero-diff per instruction; a maintainer may want to close this as superseded by #2553 once that merges.

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