[NI][SDK] Read Platform SDK names from source specs

[chris-freeman-glean]

Internal Description for reviewers:
Platform SDK generation currently hardcodes public method names in open-api, so every source endpoint addition requires a second manual mapping update in the SDK publishing repo. This PR makes the Platform transform read scio-owned x-glean-sdk metadata, validates format and uniqueness before emitting Speakeasy names, and updates generated specs from the metadata-driven local rehearsal.

PR Stack (cfreeman/api-overhaul-pr19c-platform-speakeasy):

• [NI][SDK] Add platform API generation input #102
• This PR

Companion scio PR: https://github.com/askscio/scio/pull/238861

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

Test plan:

• npm test -- tests/source-spec-transformer.test.js
• Local bundle/transform/Speakeasy SDK generation rehearsal completed successfully with the companion scio metadata source.
• 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 updates internal generation metadata for an unreleased Platform API SDK surface.

[chris-freeman-glean]

Superseded by standalone replacement PR #104.