The `ANGULARFIRE2_VERSION` build placeholder ships unreplaced in the library, and `versions.json` never reaches the compiled schematics

### Version info

**Angular:** 21.2.x **Firebase:** 12.x **AngularFire:** 21.0.0-rc.0

### How to reproduce these conditions

1. `npm pack @angular/fire@next` and inspect `fesm2022/angular-fire.mjs` — search for `ANGULARFIRE2_VERSION`.
2. Or in an app: `import { VERSION } from '@angular/fire'; console.log(VERSION.full);`

### Expected behavior

`VERSION.full` (and the underlying `Version` object in `src/core.ts`) should report the real installed version, e.g. `21.0.0-rc.0`.

### Actual behavior

`VERSION` is constructed as `new Version('ANGULARFIRE2_VERSION')` - the build-time placeholder that's supposed to be replaced with the real version before publishing, but isn't for this occurrence. The published package ships the literal string.

A related mechanism is also broken: `src/schematics/versions.json`'s `peerDependencies`/`firebaseFunctionsDependencies` data is filled in by `tools/build.ts` *after* the schematics are bundled with esbuild, which has already inlined the JSON import from the unfilled source file at compile time.
- The post-build edit to `versions.json` on disk is never read by the shipped schematic code.
- One concrete effect: `addDependencies()` in `src/schematics/common.ts` never has real version data to check against, so it can't detect or warn about conflicting dependency versions the way it appears designed to.

Will need a decision on one of these approaches:
- string-replace with a build-time guard, matching what PR #3709/#3711 just did for the schematics' own version placeholder, or
- reworking the `versions.json` inlining - the fix touches `tools/build.ts` and possibly `src/core.ts`, so I wanted to flag it and get a read before picking a direction.

從 src/core.ts 和 tools/build.ts 開始，接著檢查 src/schematics/versions.json 以及 src/schematics/common.ts 中的 addDependencies()。比較 PR #3709 和 #3711 中的建置時 placeholder 處理方式，並追蹤 JSON 何時被打包。完成的標準是：已發布的 VERSION 回報已安裝的版本，且隨附的 schematics 包含可用的相依性版本資料。

由索引模型根據 Issue 內容生成。