[firestore-bigquery-change-tracker] BigQuery client project override clobbers ADC auto-detection when bqProjectId and PROJECT_ID are both unset

Package: `@firebaseextensions/firestore-bigquery-change-tracker`, `src/bigquery/index.ts:146` on `next`.

```ts
this.bq = new bigquery.BigQuery();
this.bq.projectId = config.bqProjectId || process.env.PROJECT_ID;
```

`new BigQuery()` with no `projectId` leaves the client's internal `{{projectId}}` placeholder in place, and `@google-cloud/common` resolves it from Application Default Credentials on the first request. The assignment on line 146 overwrites that placeholder unconditionally. When the caller passes no `bqProjectId` and `process.env.PROJECT_ID` is unset, `bq.projectId` becomes `undefined`, auto-detection never runs, and every generated query references `undefined..`.

`PROJECT_ID` is an extensions-only variable. A plain Cloud Function (gen1 or gen2) never has it. The Firebase CLI sets `FIREBASE_CONFIG` and `GCLOUD_PROJECT` only.

Who is affected: any consumer of the tracker that omits `bqProjectId` and is not an extension. The firestore-bigquery-export kit is not affected, it always passes `bqProjectId` (`kits/firestore-bigquery-export/src/export-config.ts:191` falls back to `projectID.value()`). #3120 and 2.2.1 fixed the one kit-reachable variant of this (the update-view path dropped `bqProjectId`).

Suggested fix: only assign when a value exists, and prefer `GCLOUD_PROJECT` over `PROJECT_ID` as the env fallback.

```ts
const projectId = config.bqProjectId || process.env.PROJECT_ID || process.env.GCLOUD_PROJECT;
this.bq = projectId ? new bigquery.BigQuery({ projectId }) : new bigquery.BigQuery();
```

`snapshot.ts:68` has the same `bqProjectId || process.env.PROJECT_ID` fallback and should follow the same order. Both `initializeLatestView` call sites now pass `bqProjectId`, so that one is defensive only.

Found during the adversarial review of #3137. Related: #2779 proposes a `GOOGLE_CLOUD_PROJECT` fallback, which the CLI does not set either.

Start at src/bigquery/index.ts:146 and compare its client initialization with the fallback used at snapshot.ts:68. Trace how bqProjectId, PROJECT_ID, and GCLOUD_PROJECT are selected, then verify that omitted project settings preserve BigQuery ADC detection while explicit settings still select the intended project.

Written by the indexing model from the issue text.