[OpenAPI] Add Platform API to SDK spec generation

[chris-freeman-glean]

Summary

Prepare open-api to transform the Platform API once it is published by openapi-redocly automation from the clean shared-schema Platform bundle.

This PR:

• Adds generic Platform transform handling for path prefixing, schema/response prefixing, auth normalization, Platform tag-group stripping, and x-glean-sdk to Speakeasy metadata conversion.
• Removes the temporary Redocly collision-suffix workaround. The source contract now relies on scio/openapi/shared/platform_entities.yaml producing canonical names such as PersonReference, Person, and DocumentSpec.
• Adds transform unit coverage for Platform SDK metadata conversion, schema/response prefixing, clean shared schema names, auth normalization, and text/event-stream preservation.

Generated spec artifacts are intentionally excluded from this manual PR. This PR also intentionally does not add generated_specs/platform.yaml to .speakeasy/workflow.yaml; that activation belongs in a follow-up PR after automation has created the generated Platform spec.

Context

Old Platform SDK generation PRs are stale/closed: askscio/scio#237884, askscio/openapi-redocly#15, and gleanwork/open-api#102.

The active metadata-driven path comes from askscio/scio#238861 and gleanwork/open-api#104: Platform SDK names live in scio as x-glean-sdk and open-api derives Speakeasy metadata from that source contract.

The shared Platform entity schema work in scio is the prerequisite that avoids downstream Redocly suffix interpretation. open-api should transform the clean published component names, not maintain endpoint-specific mappings for names like Person-2 or DocumentSpec-2.

Expected Order

1. Land this transformer support PR.
2. Land https://github.com/askscio/openapi-redocly/pull/17.
3. Let openapi-redocly publish source_specs/platform.yaml into this repo.
4. Let .github/workflows/transform.yml generate and commit generated_specs/platform.yaml from the published source spec.
5. Open a follow-up activation PR that adds generated_specs/platform.yaml to .speakeasy/workflow.yaml and adds Platform merged-spec smoke coverage.
6. Let transform automation update .speakeasy/workflow.lock and overlayed_specs/glean-merged-spec.yaml; proceed to SDK generation only after the Platform-inclusive glean-api-specs source is available.

Validation

• Ran pnpm exec vitest run tests/source-spec-transformer.test.js tests/post_transform_smoke.test.js: 2 files, 50 tests passed.
• Ran OPEN_API_COMMIT_SHA=$(git rev-parse HEAD) pnpm run transform:source_specs with the current non-Platform source specs.
• Ran speakeasy run -s glean-api-specs --skip-upload-spec; merge and validation completed with warnings only.
• Verified the final PR diff is source/test-only: src/source-spec-transformer.js and tests/source-spec-transformer.test.js.

Out of Scope

• No generated SDK repo changes in this pass.
• No developer-site API docs changes in this pass.
• No manually committed generated OpenAPI artifacts in this PR.
• No Speakeasy activation of Platform in this PR; that happens after generated_specs/platform.yaml exists on main.