[OpenAPI] Activate Platform API SDK spec input

[chris-freeman-glean]

Summary

Activate the Platform API in the merged Speakeasy glean-api-specs source after the Platform source has been published and transformed into generated_specs/platform.yaml.

This draft PR is stacked on #112 and should stay draft until generated_specs/platform.yaml exists on main from automation.

PR Stack

1. [OpenAPI] Add Platform API to SDK spec generation #112 - transformer support for clean shared Platform schemas
2. https://github.com/askscio/openapi-redocly/pull/17 - publish bundled Platform source spec
3. This draft PR - activate the generated Platform spec in glean-api-specs

Expected Order

1. Land [OpenAPI] Add Platform API to SDK spec generation #112.
2. Land https://github.com/askscio/openapi-redocly/pull/17.
3. Let automation publish source_specs/platform.yaml and generate generated_specs/platform.yaml on main.
4. Rebase this PR onto main, then run transform automation so .speakeasy/workflow.lock and overlayed_specs/glean-merged-spec.yaml update from the Platform-inclusive input set.

Validation

• Ran pnpm exec vitest run tests/source-spec-transformer.test.js tests/post_transform_smoke.test.js: 2 files, 55 tests passed.
• Ran a local Platform-active rehearsal using the clean Redocly Platform bundle as an uncommitted source_specs/platform.yaml.
• Ran speakeasy run -s glean-api-specs --skip-upload-spec; merge and validation completed with warnings only.
• Ran pnpm exec vitest run tests/post_transform_smoke.test.js against the Platform-inclusive merged rehearsal output: 1 file, 13 tests passed.
• Verified the merged rehearsal output has the expected Platform routes/groups, clean shared Platform schema names, no x-glean-sdk, no gated-by, preserved merged tag groups, and text/event-stream for agent runs.

Out of Scope

This draft PR does not manually commit generated OpenAPI artifacts. Those should be produced by the transform workflow after the generated Platform spec exists on main.