[FLINK-39946][runtime] Unaligned checkpoint restore can mis-map union input channel state after reordering UIDed sources

[bowenli86]

What is the purpose of the change

When a Flink job restores from an unaligned checkpoint, input channel state is restored by positional input gate/channel indexes rather than by a stable edge identity.

This can break when a job has multiple source operators feeding a union and the source declarations are reordered between job versions. Even if the source operators have stable UIDs, the generated JobEdge / input gate order can still change because initial source traversal is based on transient stream node ordering. After restore, in-flight channel state from one logical source may be applied to another logical input gate.

The PR makes UID-backed union sources connect in stable UID-derived JobVertexID order, so unaligned checkpoint restore sees the same input-gate positions even if source declaration order changes.

Brief change log

• Updated job graph generation to order UID-backed union source entry points by stable UID-derived JobVertexID instead of declaration/node-id order.
• Applied the same stable ordering in the adaptive graph reconnect path so lazily created downstream vertices get consistent input-gate order too.
• Preserved legacy ordering for non-UID and uidHash-only source heads.

Verifying this change

This change added tests and can be verified as follows:

• Added regression coverage that reversed UID-backed union source declaration order produces the same sink input order.
• Added regression coverage for mixed UID-backed union inputs plus an unrelated source.
• Added coverage that uidHash-only union inputs keep declaration order.

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)

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CI report:

• d4dd575 Azure: SUCCESS

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[bowenli86]

this is related to unaligned checkpoint, @pnowojski maybe you can help review?