[FLINK-39394][web] Fix job overview metrics broken when a vertex is finished

[Izeren]

What is the purpose of the change

When a streaming job has a mix of RUNNING and FINISHED vertices, the job overview page shows "N/A" for backpressure, busyness, and data skew metrics on all vertices — including running ones. A single finished
vertex's empty metrics response causes a TypeError that kills the entire forkJoin, discarding metrics for all nodes.

Brief change log

- Guard against missing metric keys in `mergeWithBackPressureAndSkew` before accessing `.max`/`.skew`
- Add per-node `catchError` in both `mergeWithBackPressureAndSkew` and `mergeWithWatermarks` so a single vertex failure does not discard metrics for all other vertices

Verifying this change

This change is a trivial rework / code cleanup without any test coverage.

Manually verified by running a STATEMENT SET with one bounded (VALUES) and one unbounded source inserting into the same sink. The bounded source vertex finishes while the unbounded one keeps running. Without the
fix, all vertices show "N/A"; with the fix, running vertices display real metrics.

Does this pull request potentially affect one of the following parts:

- Dependencies (does it add or upgrade a dependency): no
- The public API, i.e., is any changed class annotated with `@Public(Evolving)`: no
- The serializers: no
- The runtime per-record code paths (performance sensitive): no
- Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: no
- The S3 file system connector: no

Documentation

- Does this pull request introduce a new feature? no
- If yes, how is the feature documented? not applicable

[flinkbot]

CI report:

• bb2c7a1 Azure: FAILURE

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[pnowojski]

I have verified this manually

[pnowojski]

@flinkbot run azure