[NI][SDK] Generate Platform SDK names from source metadata

[chris-freeman-glean]

Internal Description for reviewers:
Platform SDK generation currently depends on open-api-owned endpoint mapping when the intended contract is owned by scio's Platform source specs. This PR prepares open-api to consume scio-owned x-glean-sdk metadata generically in the source-spec transform, but intentionally does not add Platform to the published glean-api-specs merge input yet. That lets us land the transform plumbing and validation path without exposing experimental, feature-gated Platform methods in the public Python, TypeScript, Go, and Java SDKs.

The Platform source spec, generated specs, merged Speakeasy output, and lockfile are intentionally left to the existing scio → redocly → open-api and transform workflows. A TODO remains in .speakeasy/workflow.yaml marking the exact generated_specs/platform.yaml input to add once Platform endpoints are ready to appear in generated clients.

Companion scio PR: https://github.com/askscio/scio/pull/238861
Supersedes stacked draft PR: #103
Supersedes manual-mapping PR: #102

Context/Jira (Mandatory if external):
N/A

Test plan:

• npm test
• Local generated-flow rehearsal: temporarily copied the bundled Platform spec produced from askscio/scio#238861 into source_specs/platform.yaml, ran OPEN_API_COMMIT_SHA=$(git rev-parse HEAD) pnpm run transform:source_specs, then speakeasy run -s glean-api-specs --skip-upload-spec; workflow-owned inputs/outputs were removed/restored afterward so generated files stay out of this PR.
• Synthetic proof on standalone branch: adding a new operation with unique x-glean-sdk metadata mapped to the expected Speakeasy SDK method without code changes; removing that metadata failed the transform with an actionable error.
• Local all-client rehearsal: temporarily pointed Python, TypeScript, Go, and Java SDK generation at a locally merged Platform-inclusive spec to verify the transform can generate all four language surfaces once publishing is enabled. The workflow now intentionally leaves Platform out of the published merged spec until the endpoints are ready for SDK exposure.
• pnpm exec vitest run tests/source-spec-transformer.test.js
• Need review from all reviewers (default: 1)

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Release Notes (go/relnotesfaq)

Change Categories (mandatory - check all that apply)

• Internal (This PR will not be included in the external release notes)
  • Flag-gated development/Internal fix
• External (Choose one if applicable. A detailed external description is mandatory)
  • Bug Fixes/Enhancements
  • Security or Permissions related change
  • Feature launch
  • UI change
  • 3rd party LLM integration change
  • External API breaking change
  • Scrubbed logs change
• Platform (Choose one if applicable. Choose at least one other category from Internal or External)
  • AWS only change
  • Azure only change
  • GCP only change

• Document Updates Needed

(DO NOT include confidential details. Should be at least 40 characters long.)
External description for customers:
No customer-visible behavior change; this lands internal Platform SDK transform plumbing without publishing the unreleased Platform API methods into generated clients.