fix(sdk): resolve 0.4.0 premortem and reconcile PRs #363 and #368

[davideast]

Summary

Follow-up to the 0.4.0 5-PR stack (#369–#373) addressing all 4 verified Tigers identified in the post-merge Pre-Mortem review and reconciling open PRs #363 and #368:

1. Tiger 1 ([HIGH]): Restore publishConfig.tag: "latest" for 0.4.0

   • Updates packages/sdk/package.json publishConfig.tag from "next" back to "latest" so npm install @google/stitch-sdk installs 0.4.0.
   • Restores the strict assert.strictEqual(tag, "latest") check for non-prerelease versions in scripts/publish-readiness.ts and updates RELEASING.md.

2. Tiger 2 ([HIGH]): Prevent await stitch.close() from permanently bricking the stitch singleton (CLIENT_CLOSED)

   • Adds public get closed(): boolean on StitchToolClient.
   • Updates singleton.ts so await stitch.close() clears _client, _clientCacheKey, and _stitch without eagerly constructing a client when called before first use (preventing ZodError when credentials aren't set yet), and ensures getOrCreateClient() replaces any closed client.

3. Tiger 3 ([MEDIUM]): Preserve 0.3.5 modelId union literals (GEMINI_3_PRO, GEMINI_3_FLASH, GEMINI_3_1_PRO)

   • Preserves 0.3.5 modelId enum values alongside 0.4.0 literals (GEMINI_3_8_FLASH, GEMINI_3_5_FLASH_LITE) during load-time and runtime schema repair so TypeScript callers and @google/stitch-sdk/ai / /adk Zod tool schemas remain backward-compatible.

4. Tiger 4 ([MEDIUM]): Prevent cyclic TypeError on JSON.stringify(screen) / JSON.stringify(generation)

   • Defines this.client as non-enumerable (enumerable: false) in generated domain class constructors (scripts/generate-sdk.ts) and EntityManager (packages/sdk/src/entity-manager.ts), breaking the client.entities.client === client enumeration cycle and keeping client out of { ...screen } spreads.

5. Reconcile PR #368 & PR #363:

   • PR #368: Adds collectDefPool() and multi-pass transitive $ref resolution in packages/sdk/src/schema-repair.ts and exports repairToolSchemas, repairSchema, and collectDefPool from the root entry point.
   • PR #363: Normalizes transient fetch/socket errors (fetch failed, ECONNRESET, socket, other side closed) into StitchError(NETWORK_ERROR), marks isConnected = false, and reconnects inside the callTool retry loop for idempotent reads (get_* / list_*).