feat(schematics): rewrite Vertex AI imports to AI Logic on ng update

[armando-navarro]

Adds an ng update migration that moves a workspace off the Vertex AI module onto Firebase AI Logic, and a guide for upgrading from AngularFire 20 to 21.

Background

AngularFire 21 renamed the Vertex AI module to Firebase AI Logic. The @angular/fire/vertexai entry point (and the older @angular/fire/vertexai-preview) were removed in favor of @angular/fire/ai, and the exported symbols were renamed. A project upgrading from 20 that used Vertex AI would fail to compile until it updated those imports by hand.

What this does

• Extends the existing v21 migration (the one that aligns the firebase dependency) to also rewrite Vertex AI imports and their usages to AI Logic. It parses each source file with the TypeScript compiler and edits only real references, leaving strings, comments, and unrelated identifiers that happen to share a name untouched. It handles named imports and aliases, namespace imports in value and type position, bare local re-exports, and shorthand properties.
• Adds docs/version-21-upgrade.md, a note in docs/ai.md, and a README link.
• Adds typescript to the schematics esbuild externals, so the compiler is resolved from the workspace at ng update time rather than bundled into the package. This matches how Angular's own migrations ship and keeps the package from growing by several megabytes.

Symbol map

Before (@angular/fire/vertexai)	After (@angular/fire/ai)
getVertexAI	getAI
provideVertexAI	provideAI
VertexAI	AI
VertexAIInstances	AIInstances
vertexAIInstance$	AIInstance$
VertexAIModule	AIModule

getGenerativeModel and getImagenModel keep their names.

Known limitation

The rewrite matches by name, so a local variable that shadows an imported name with the same spelling can be mis-renamed. ng update always shows its changes as a diff to review, so this surfaces on inspection.

Verification

Unit specs cover each kind of code the migration can run into:

• named imports, and aliased imports where only the imported name changes
• namespace imports, in both value position (ns.getVertexAI()) and type position (ns.VertexAI)
• shorthand properties ({ getVertexAI }) and bare local re-exports (export { VertexAI })
• unchanged symbols, a file with nothing to rewrite, and an idempotent second run

I also ran the migration for real, not just in specs:

• scaffolded a fresh Angular 20 app that imports from @angular/fire/vertexai
• ran ng update and confirmed every import and usage rewrote to @angular/fire/ai
• confirmed firebase aligned to ^12.4.0, and that a second run made no changes

Refs #3686