docs: Remove old x-model-router-rationale header

[grahamking]

We don't provide free-form routing info to the user, just the model that
was selected. The free-form info goes in logs.

Signed-off-by: Graham King grahamk@nvidia.com

Summary by CodeRabbit

• Documentation
  • Removed documentation for the x-model-router-rationale response header from the observability guide.

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• docs/routing_algorithms/stage_router_routing.md

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• docs/routing_algorithms/stage_router_routing.md

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Walkthrough

The routing documentation removes the x-model-router-rationale response header and its human-readable routing reason.

Changes

Routing documentation

Layer / File(s)	Summary
Remove obsolete routing header documentation docs/routing_algorithms/stage_router_routing.md	The documentation no longer describes the x-model-router-rationale response header.

Estimated code review effort: 1 (Trivial) | ~2 minutes

Merge Risk: ⚪ Minimal · up to c62a1

This change only removes an outdated documentation header and does not alter product behavior; no actionable merge-blocking risk remains.

Poem

I’m a rabbit with a tidy pen,
One stale header leaves the den.
The routing guide is clear and bright,
No extra reason in sight.
Hop, hop—docs now match the flight!

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
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.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the removal of the outdated x-model-router-rationale header documentation.

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[ayushag-nv]

@grahamking what other headers still exists which are emitted with model routing ? is it just the selected model ? I remember earlier I added bunch of 4-5 headers but that was long ago in python version

[grahamking]

@grahamking what other headers still exists which are emitted with model routing ? is it just the selected model ? I remember earlier I added bunch of 4-5 headers but that was long ago in python version

Yes, just x-model-router-selected-model. I suspect none of the others came over to Rust, and the docs weren't updated.

[github-actions]

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[ayushag-nv]

lgtm