fix(firestore-incremental-capture-pipeline): adjustDate shifts the restore point on non-UTC JVMs

### [REQUIRED] Step 2: Describe your configuration

- Extension name: `firestore-incremental-capture`
- Extension version: any; concerns the `firestore-incremental-capture-pipeline` Dataflow job
- Configuration values: only manifests when the pipeline JVM's default timezone is not UTC

### [REQUIRED] Step 3: Describe the problem

`Utils.adjustDate` (firestore-incremental-capture-pipeline/src/main/java/com/pipeline/Utils.java) round-trips the restore timestamp through a zone-less string:

```java
return Instant.parse(providedDate.toString("yyyy-MM-dd'T'HH:mm:ss.SSS"));
```

`providedDate.toString(...)` renders in the JVM's default zone with no offset suffix, and `Instant.parse` interprets the zone-less string as UTC. On a JVM whose default timezone is not UTC, the restore point silently shifts by the zone offset, so the replay window cuts at the wrong moment.

In practice Dataflow workers run UTC, so current jobs are unaffected - this is a latent correctness bug, not a live incident. It also surfaces in any local or test execution of the pipeline in a non-UTC environment.

#### Suggested fix

Use `providedDate.toInstant()` (and `now.minusDays(7).toInstant()` in the 7-day clamp branch) instead of the format/parse round trip. No wire-format or schema impact.

#### Related

Raised by review tooling on firebase/extensions#2938 while the pipeline was vendored there; the pipeline's canonical home is this repo, so tracking it here. Low severity given UTC workers; worth folding into the next pipeline release after v0.1.0 alongside #1147.

Open firestore-incremental-capture-pipeline/src/main/java/com/pipeline/Utils.java and start at Utils.adjustDate. Verify how restore timestamps behave when the JVM default timezone is not UTC, then confirm the pipeline preserves the supplied instant, including the seven-day clamp branch, without changing wire formats or schemas.

Written by the indexing model from the issue text.