fix(ai): drop the getImagenModel wrapper, removed in firebase 12.18

[armando-navarro]

Checklist

• Issue number for this PR: @angular/fire/ai fails to build against firebase 12.18, which removed getImagenModel #3749
• Docs included?: yes, the upgrade guide and docs/ai.md
• Test units included?: yes, a migration spec for the new removed-symbol guidance (227 specs, 0 failures)
• In a clean directory, yarn install, yarn test run successfully?: yes

Description

Fixes #3749.

Firebase shut the Imagen models down in August 2026 and removed the Imagen API in @firebase/ai 2.15.0, which ships in firebase 12.18.0. The ai entry point imports getImagenModel, so any app whose install resolves firebase 12.18.0 fails to build.

The fix

• The generator excludes getImagenModel from the wrapped named imports (a null override in tools/build.ts).
• Installs whose firebase still includes the symbol (12.17 and older) keep compiling, because the entry point re-exports everything from firebase/ai with export *. That is compile-time grace only: the models are shut down, so the calls fail at the service on any firebase version.
• The package deliberately keeps building against firebase 12.4.0, the lowest version the published ^12.4.0 range allows. That way every symbol the generated file imports by name exists in every firebase version a user's install can resolve.

Verification

• A sweep of every wrapped @firebase package between the 12.4.0-mapped and 12.18.0-mapped versions confirmed getImagenModel is the only export removed in that window.
• A typecheck of all generated named imports against a real firebase 12.18.0 install passes with this change and fails without it.
• 227 specs, 0 failures.

Also in this change

• The ng update Vertex AI migration treats getImagenModel as a removed symbol, leaving the import in place and logging guidance to move image generation to the Gemini image models through getGenerativeModel, with a new spec covering it.
• The upgrade guide documents the removal, and its migration section is restructured into labeled cases and subheadings.
• docs/ai.md no longer advertises Imagen models.

[tyler-reitz]

Verified independently: the named imports of all 14 generated firebase.ts files typecheck against a real firebase 12.18.0 install, and re-adding getImagenModel reproduces the TS2305 from #3749; regenerating every entry point with this branch's tools/build.ts produces zero drift against the committed files; the new migration spec goes red with the tables.ts entry removed; and the published rc tarball keeps export * from 'firebase/ai' verbatim in the shipped .d.ts and fesm, so the fix carries through to the artifact users install.

One note, not a change request: on firebase 12.17 and older, getImagenModel is still reachable through the star export but now unwrapped, where the zone-wrapped named export used to shadow it (ran both shapes to confirm the precedence). Since every call fails at the shut-down service, the only effect is the rejection settling outside the Angular zone, consistent with the compile-time-grace framing in the description.