fix(providerroutes): degraded stitch must not emit the version segment twice

[damonleelcx]

⚠️ Post-hoc review PR. This commit is already on develop-v1.0.5 — it was pushed there directly. To make it reviewable as an isolated diff, the base here is a snapshot branch at its parent (a2e8936) rather than develop-v1.0.5, which already contains it. Do not merge. Review, comment, then both review/* and review-base/* branches can be deleted; any change lands as a new commit on develop-v1.0.5.

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The bug

A host absent from provider_routes takes the literal-prepend branch. When the stored base_url already ends in the exact segment the client sends, it was emitted twice:

https://www.cun.ai/v1  +  /v1/chat/completions
  ->  https://www.cun.ai/v1/v1/chat/completions      (upstream 404)

This is not an edge case. The mixed-version manifest carries 19 real vendor endpoints in this shape — vercel ai-gateway, aihubmix, 302.ai, cerebras, fireworks, hunyuan, minimax, mistral, moonshot, sambanova, stepfun, together, z.ai. Those vendors document their base_url with /v1 on it, so the user who pasted the documented URL got a broken route and the one who happened to trim it got a working one.

Same /v1/v1 poison already fixed on the OAuth path (stitchOAuthRequestURL) and for known rows (2026-08-03); this join was the last one left.

What is deliberately NOT changed

The degraded path stays shape-agnostic. Its existing note is explicit, and it stands:

🚫 The DEGRADED path … is untouched on purpose. … a private gateway stored as https://gw.example serving /v1/chat/completions would break the moment we swallowed its /v1. Deciding for a vendor we know is not the same act as deciding for one we do not.

For an unknown vendor we genuinely cannot tell a mount prefix (https://gw.example/proxy serving /proxy/v1/chat/completions) from a complete API root — so we must not decide. This change decides nothing about shape. It only refuses to emit the same segment twice; /v1/v1 is wrong under every reading. The segment still survives — contributed by basePath instead of by reqPath — so the bare-host case that note protects is byte-identical.

trailingVersionSegment shares trimLeadingVersionSegment's digits-only strictness, so /v1beta and /v1abc are never treated as duplicates of /v1.

Behaviour matrix

stored base_url	client	result
https://www.cun.ai	/v1/chat/completions	/v1/chat/completions	✅ unchanged
https://gw.example/proxy	/v1/chat/completions	/proxy/v1/chat/completions	✅ unchanged (mount prefix)
https://relay/api/v3	/v1/chat/completions	/api/v3/v1/chat/completions	✅ unchanged (different segment ≠ duplicate)
https://gw.example/v1beta	/v1/models	/v1beta/v1/models	✅ unchanged (not digits-only)
https://www.cun.ai/v1	/v1/chat/completions	/v1/chat/completions	🔧 fixed
https://relay/api/v3	/v3/chat/completions	/api/v3/chat/completions	🔧 fixed

Fences

stitch_unknown_host_dedup_test.go — 11 cases. 4 pin the collapse; 6 pin that every other shape stays byte-identical — those carry the weight, they guard the "do not decide for an unknown vendor" rule; 1 pins the strictness helper.

Mutation-verified: removing the guard reproduces www.cun.ai/v1/v1/chat/completions exactly.

Consumers: this changes what aikey-proxy dials for affected credentials. The 19 manifest rows it legitimately moves are registered in the sibling aikey-proxy review PR via stitchAlgorithmChangedRows; the field-behaviour manifest is deliberately left untouched.

Not done

D-17's fail-loud half. provider_routes is an embedded fixed list with no user-extension mechanism, so rejecting unknown hosts would break every third-party relay outright — a product-capability decision needing a relay-registration channel first. Remains 单独立项.

Refs: workflow/CI/bugfix/2026-08-15-degraded-stitch-duplicates-version-segment.md

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