fix(sse): propagate malformed Anthropic JSON

[404-Page-Found]

Summary

• propagate malformed JSON errors from Anthropic message_start and content_block_delta SSE frames
• keep the error provider-specific and consistent with the OpenAI SSE parser
• add parser regressions for both event types
• add an end-to-end streaming regression that verifies the parser error releases the response body

Fixes #307

Verification

• Reviewed the two-file diff against main
• Local test execution was unavailable because this environment cannot resolve github.com; repository PR checks should provide the executable verification

[coderabbitai]

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📥 Commits

Reviewing files that changed from the base of the PR and between 65718e4 and 23cf8f1.

📒 Files selected for processing (2)

• src/providers/sse.ts
• tests/stream-sse.test.mjs

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[cubic-dev-ai]

All reported issues were addressed across 2 files

<sub>Shadow auto-approve: would not auto-approve because issues were found.

Re-trigger cubic</sub>

[sonarqubecloud]

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[cubic-dev-ai]

0 issues found across 2 files (changes from recent commits).

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

<sub>Shadow auto-approve: would auto-approve. This PR fixes a bug where malformed Anthropic SSE JSON was silently ignored and instead propagates a provider-specific parse error, with parser and end-to-end regression tests. The change preserves non-delta/message_start behavior, adds null handling, and is tightly scoped to parser error handling.

Re-trigger cubic</sub>