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.

Bắt đầu với src/core.ts và tools/build.ts, sau đó kiểm tra src/schematics/versions.json và addDependencies() trong src/schematics/common.ts. So sánh cách xử lý placeholder trong thời gian build ở các PR #3709 và #3711, đồng thời theo dõi thời điểm JSON được đóng gói. Hoàn thành có nghĩa là VERSION được phát hành báo cáo phiên bản đã cài đặt và các schematics được phân phối chứa dữ liệu phiên bản dependency có thể sử dụng.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.