fix: skip metadata-only SSE chunks instead of aborting stream in StreamingResponseAggregator

[anishesg]

Link to Issue or Description of Change

Closes: #782

Vertex AI occasionally emits a leading SSE chunk that contains only response metadata (createTime, modelVersion, usageMetadata, etc.) with no Candidates field. This happens consistently with gemini-3-flash-preview + googleSearch grounding and caused roughly 40–50% of streaming calls to fail with "empty response", even though the actual content arrived in subsequent chunks.

The root cause is in internal/llminternal/stream_aggregator.go in ProcessResponse: when len(genResp.Candidates) == 0, the function called yield(nil, fmt.Errorf("empty response")) and returned, aborting the iterator. The fix changes this to a silent return, allowing the aggregator to skip metadata-only chunks and continue processing subsequent chunks that carry real content. This matches the behavior of adk-python's PROGRESSIVE_SSE_STREAMING path which handles identical chunks gracefully.

The unused fmt import was also removed as a consequence.

Testing Plan

Unit Tests

• Added TestMetadataOnlyChunkDoesNotAbortStream in stream_aggregator_test.go that feeds a metadata-only chunk (no Candidates) followed by a real content chunk and asserts the aggregated response contains the expected text without any error.
• All existing stream aggregator unit tests continue to pass.

go test ./internal/llminternal/... -run "TestProgressiveSSE|TestStreaming|TestFinishReason|TestMetadata" -v
--- PASS: TestProgressiveSSEStreamingFunctionCallArguments (0.00s)
--- PASS: TestProgressiveSSEPreservesThoughtSignature (0.00s)
--- PASS: TestProgressiveSSEHandlesEmptyFunctionCall (0.00s)
--- PASS: TestStreamingFCChunkWithWillContinueButNoPartialArgs (0.00s)
--- PASS: TestProgressiveSSEStreamingFunctionCalls (0.00s)
--- PASS: TestProgressiveSSEPreservesPartOrdering (0.00s)
--- PASS: TestMetadataOnlyChunkDoesNotAbortStream (0.00s)
--- PASS: TestFinishReasonUnexpectedToolCallPreservesErrorCode (0.00s)
PASS

Manual E2E Tests

The reproduction script from the issue (10 runs of gemini-3-flash-preview + googleSearch with StreamingModeSSE) now produces 10/10 successes with this patch applied, matching the behavior of StreamingModeNone and adk-python.

Checklist

• I have read the CONTRIBUTING.md file.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added unit tests that prove my fix is effective or that my feature works.
• All unit tests pass locally.
• I have completed manual E2E testing.
• Any dependent changes have been merged and published.

Fixes #782