**Changes**
- Added a local watermark-specific message-parameters subclass to register the new optional query flag: `JobVertexWatermarksMessageParameters.java`.
- Added the new optional boolean query parameter `includeMissing`, defaulting to `false`, in `IncludeMissingQueryParameter.java`.
- Updated `JobVertexWatermarksHandler.java` to:
  - read `includeMissing` from the request,
  - preserve the existing behavior when it is absent or `false`,
  - emit one `Metric` per subtask even when the watermark metric is missing, using the existing metric id and a `null` value, when `includeMissing=true`.
- Updated `JobVertexWatermarksHeaders.java` so the endpoint advertises the new behavior.
- Expanded `JobVertexWatermarksHandlerTest.java` to cover:
  - default behavior,
  - partial availability,
  - explicit `includeMissing=true`,
  - explicit `includeMissing=false`.

**Tests**
- `mvn -pl flink-runtime -P!java21 -Dtest=JobVertexWatermarksHandlerTest -Dcheckstyle.skip=true -Denforcer.skip=true test` passed.

**Remaining uncertainty**
- Validation was limited to the focused handler test in this JDK 25 environment. I did not run broader `flink-runtime` or end-to-end REST tests, so wider integration behavior remains unverified.