fix(llm): emit role once in chat streams

[rmccorm4]

Summary

• Advance the chat delta generator's message counter so it emits delta.role only on the first generated chunk.
• Deduplicate roles per choice after tool and reasoning parser transformations, which can synthesize new role-bearing chunks.
• Preserve the initial role-only chunk instead of filtering it as empty.

Both layers are required: fixing only the base generator does not cover parser-generated chunks, while boundary-only normalization would hide the generator defect.

Validation

• cargo fmt --all -- --check
• cargo test -p dynamo-llm --lib test_role_is_emitted_only_in_first_delta
• cargo test -p dynamo-llm --lib test_deduplicate_stream_roles_preserves_only_first_role_per_choice

Tracking

Internal: DIS-2622

Summary by CodeRabbit

• Bug Fixes
  • Improved chat response streaming so assistant role information is preserved when received in a standalone update.
  • Prevented repeated role information from appearing in subsequent streamed chunks.
  • Improved handling of multiple choices so each choice emits its role correctly and independently.
  • Updated streaming behavior to avoid incorrectly discarding meaningful role-only updates.

[coderabbitai]

Walkthrough

Chat streaming now treats role-only deltas as meaningful. It preserves the first role per choice, removes repeated roles before empty-chunk filtering, and updates delta-generation and stream-processing tests.

Changes

Chat role streaming

Layer / File(s)	Summary
First-delta role generation lib/llm/src/protocols/openai/chat_completions/delta.rs	DeltaGenerator::create_choice increments its message counter after creating a choice. Tests verify that only the first streaming choice includes the assistant role.
Chat SSE role deduplication lib/llm/src/http/service/openai.rs	Chat stream emptiness checks include roles. The handler tracks emitted roles per choice and removes repeated roles before SSE conversion. Tests retain role-only chunks and verify per-choice deduplication.

Estimated code review effort: 3 (Moderate) | ~20 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the primary change: emitting the chat stream role only once.
Description check	✅ Passed	The description explains the changes, validation commands, and tracking issue, with sufficient detail for review despite different section headings.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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[indrajit96]

LGTM!