Keep the SDK-only caveat out of variant summaries

[chris-freeman-glean]

Problem

platform-chat-create-stream is titled "SDK-only logical operation. HTTP clients must call the base path; the URL fragment is not sent." instead of the name of the operation.

prefixSdkOnlyDocs in src/source-spec-transformer.js prepended that caveat onto both the summary and the description of every derived fragment variant. The summary is what the developer site renders as the page title and H1, so the boilerplate replaced the title.

Change

applySdkVariantDocs replaces prefixSdkOnlyDocs:

• The summary is never prefixed. A variant may declare its own summary; otherwise the parent summary carries over unchanged.
• The caveat still leads the description, where it reads as prose rather than a heading. It describes the fragment path itself rather than the source text, so it applies even when a variant overrides the description.
• platformSdkVariantKeys gains summary and description, each validated as a non-empty string when present.

Live verification against the Platform variant summary

Verified end to end by feeding openapi/public/platform/chat.yaml from askscio/scio at commit 10b689f4fb17c052f2b09740ded75c75d04cb072 (blob b8dd28b6cb5e82bcce81e3091440ea0cfc1981f2, PR askscio/scio#285668, still open) into this branch's transformer at c3059f7929f404db992f1101cdf8cc427a45b4d0, then running the glean-developer-site Platform OpenAPI pipeline (openapi-capitalize-language → openapi-server-url → docusaurus gen-api-docs platform) and rendering locally.

Operation	H1 / sidebar
/api/chat#stream (platform-chat-create-stream)	Create a streaming chat response
/api/chat (platform-chat-create)	Create a chat response

The SDK-only caveat now appears only in the streaming page's description prose, never in its title.

Sequencing

This PR is code and tests only. No final_specs, Speakeasy, or developer-site output is regenerated here, and nothing above was published.

Platform's per-variant summary: Create a streaming chat response is not merged yet. Once it lands, the streaming page picks up its own distinct title through variant.summary with no further change to this transformer, and the spec republish plus developer-site redeploy can follow.

Until then, the streaming page inherits the parent's "Create a chat response" — the same title as the non-streaming page, but a real operation title rather than boilerplate.

One follow-up lives outside this repo: sidebars.ts in glean-developer-site still carries a hardcoded label containing the old boilerplate for this doc id, and pnpm sidebar:fix does not rewrite label text. It is inert today because the generated page's sidebar_label frontmatter wins at render time (confirmed in the built sidebar payload and the screenshots above), but it is stale text worth cleaning up there.

Testing

pnpm test — 120 passed across 5 files. pnpm lint — clean.

• Updated the two tests that asserted the concatenated summary.
• Added a test for variant summary and description overrides.
• Added a test rejecting a non-string variant summary.