[REVIEW ONLY — DO NOT MERGE] fix(providerroutes): degraded stitch must not emit the version segment twice

[damonleelcx]

🔴 DRAFT ON PURPOSE — do not mark ready, do not merge.
This commit is already on develop-v1.0.5 (18eca55). GitHub refuses a PR against develop-v1.0.5 — "No commits between develop-v1.0.5 and review/…" — because a PR proposes commits the base lacks, and the base already has this one. The base here is therefore a snapshot at its parent (a2e8936), purely so the diff is readable and commentable.
Merging accomplishes nothing (it would move two throwaway branches). An earlier version of this PR was merged unread within two minutes, which is why this one is a draft. Anything review turns up lands as a new commit and a normal PR against 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)

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 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 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 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.

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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