[NI][SDK] Add platform API generation input

[chris-freeman-glean]

Why

The new Platform API surface needs to publish into generated SDKs without leaking SDK naming policy into scio's source OpenAPI specs. Scio's operationId drives Go server codegen and should remain a stable server/API identity; the SDK publishing repo should own the public SDK namespace and method shape.

The SDK repo also merges several OpenAPI specs together, so Platform schemas and shared responses need deterministic namespacing to avoid collisions with the existing REST and indexing contracts. Public SDK inputs also must not expose scio runtime config details such as feature flag names.

What Changes

This PR adds Platform API as an SDK generation input and introduces a Platform transform that explicitly maps scio operation IDs to SDK names. The current mappings are platform-search -> platform.search.query and platform-tools-list -> platform.tools.list, which generate client.platform.search.query(...) and client.platform.tools.list(...) while preserving source operation IDs.

Platform schemas and shared responses are prefixed, Platform auth is normalized to the existing APIToken scheme, and unmapped Platform operations or namespace collisions fail during transform instead of silently producing a broken merged spec. There is no Platform overlay file in this PR; the default path is transform-driven and the SDK surface is reviewed through the explicit mapping.

The regenerated Platform public specs preserve x-glean-experimental.id and introduced for both endpoints but no longer include x-glean-experimental.gated-by. That stripping happens at the scio Redocly public-export boundary before these SDK artifacts are generated.

Cross-Repo Stack

1. scio source contract and private metadata stripping: https://github.com/askscio/scio/pull/237884
2. Redocly bundle publication: https://github.com/askscio/openapi-redocly/pull/15
3. SDK spec transform and explicit SDK mapping: [NI][SDK] Add platform API generation input #102 (this PR)

Merge order should follow the stack above so this repo consumes the Platform bundle produced by the previous steps.

Test Plan

• pnpm run transform:source_specs
• speakeasy run -s glean-api-specs --skip-upload-spec
• pnpm vitest run tests/source-spec-transformer.test.js tests/post_transform_smoke.test.js
• Verified gated-by is absent from source_specs/platform.yaml, generated_specs/platform.yaml, and overlayed_specs/glean-merged-spec.yaml, while experimental id and introduced remain for /api/search and /api/tools.
• Verified transformed and merged specs contain /api/search as platform.search.query and /api/tools as platform.tools.list, with Platform-prefixed models and responses.