feat(frontend): expose detailed finish reason

[Aphoh]

Summary

• Expose Dynamo's canonical internal finish reason through opt-in nvext.detailed_finish_reason.
• Keep standard OpenAI finish_reason values unchanged, so cancellation remains stop.
• Preserve the extension through streaming and non-streaming aggregation for /v1/chat/completions and /v1/completions.
• Use a typed NvExtResponseInput boundary for reusable response metadata construction.
• Document the new nvext.extra_fields selector.

SGLang 0.5.18 defaults pause_generation to abort mode and emits an abort finish reason. Dynamo parses both abort and cancelled as FinishReason::Cancelled. Clients can now request detailed_finish_reason and receive "cancelled" without introducing a non-standard OpenAI finish reason.

Closes #14418

Validation

• cargo test -p dynamo-llm detailed_finish_reason --lib
• cargo test -p dynamo-llm protocols::common::extensions::tests --lib
• cargo test -p dynamo-llm test_multiple_deltas_merge_nvext_fields --lib
• cargo clippy -p dynamo-llm --lib --tests -- -D warnings
• cargo fmt --all -- --check
• python3 docs/fern/scripts/docs_lint.py --scan docs (0 errors; 8 pre-existing warnings)
• Live NVIDIA B200 test with SGLang 0.5.18 and Qwen3-0.6B:
  • /v1/completions: finish_reason: "stop", nvext.detailed_finish_reason: "cancelled"
  • /v1/chat/completions: finish_reason: "stop", nvext.detailed_finish_reason: "cancelled"
  • Both requests were interrupted by calling /engine/pause_generation without a mode, exercising SGLang's default mode="abort".

[coderabbitai]

Walkthrough

The change adds optional backend termination_status support to NVExt responses. OpenAI chat and completion responses propagate this value while preserving existing finish_reason mappings. Aggregation tests and NVExt documentation cover the new field.

Changes

Termination status propagation

Layer / File(s)	Summary
NVExt termination-status contract lib/llm/src/protocols/common/extensions.rs	NVExt response selection and construction support optional termination_status. Tests cover field selection, forwarding, and final-response behavior.
OpenAI termination-status propagation lib/llm/src/protocols/openai/chat_completions/delta.rs, lib/llm/src/protocols/openai/completions/delta.rs	Chat and completion delta processing passes backend termination status to NVExt while retaining the existing OpenAI finish-reason mapping. Regression tests cover cancelled responses.
Aggregated output and documentation lib/llm/src/protocols/openai/chat_completions/aggregator.rs, lib/llm/src/protocols/openai/completions/aggregator.rs, docs/fern/pages/developer-guide/additional-resources/nvidia-request-extensions-nvext.md	Aggregation tests preserve termination_status. Documentation lists the field, canonical values, and selection behavior.

Priority: ➖ Normal

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

Severity of issue fixed: Medium

Merge Risk: 🟡 Moderate · up to 1b3c2

The new optional termination-status field can misreport why a completion ended when a request returns multiple choices with different terminal states. This should be constrained to single-choice requests or represented per choice before merge.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	The changes satisfy the linked issue [#14418] by preserving backend cancellation information through opt-in nvext.termination_status while retaining standard OpenAI finish_reason values. The implement…
Out of Scope Changes check	✅ Passed	The code, tests, and documentation changes directly support the linked issue and stated objectives. No unrelated changes are evident.
Docstring Coverage	✅ Passed	Docstring coverage is 91.30% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 23 functions across 5 files. (1 skipped: 1 …
Title check	✅ Passed	The title clearly describes the primary behavior change: exposing a detailed backend finish reason. The frontend scope and wording do not exactly match the nvext.termination_status implementation,…
Description check	✅ Passed	The description provides a clear summary, implementation details, validation commands, live test results, and a linked issue. It does not use all template headings, especially `Where should the review…

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[coderabbitai]

Actionable comments posted: 1

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Inline comments:
In `@lib/llm/src/protocols/common/extensions.rs`:
- Around line 800-804: Update the termination_status assignment in the response
aggregation flow so it is emitted only when the request contains exactly one
choice; otherwise return None rather than reusing
termination_status_from_backend. Preserve the existing single-choice behavior
and do not aggregate one backend status across multiple choices.

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📥 Commits

Reviewing files that changed from the base of the PR and between 4a205cb and 1b3c21f.

📒 Files selected for processing (6)

• docs/fern/pages/developer-guide/additional-resources/nvidia-request-extensions-nvext.md
• lib/llm/src/protocols/common/extensions.rs
• lib/llm/src/protocols/openai/chat_completions/aggregator.rs
• lib/llm/src/protocols/openai/chat_completions/delta.rs
• lib/llm/src/protocols/openai/completions/aggregator.rs
• lib/llm/src/protocols/openai/completions/delta.rs

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[github-actions]

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