[Flink] Migrate streampark-flink-proxy from Scala to Java

[shangeyao]

Summary

Parent epic: #4408
Stage 2 — Job deployment (2/4)

Migrate streampark-flink-proxy from Scala to Java with no behavior changes (Stages 1–2) unless explicitly in Stage 3 scope.

Module role

Shims proxy — FlinkShimsProxy, ChildFirstClassLoader, per-Flink-version isolation and dispatch.

Dependencies

• Depends on: [Flink] Migrate streampark-flink-packer from Scala to Java #4449 (streampark-flink-packer migration must merge first)
• Blocks: Stage 2.3

PRs within Stages 1–2 are sequential — do not start until the dependency issue is merged.

Constraints

• Follow Java architecture and idioms; keep package paths where possible
• Each new Java class should have tests (or document why not) — PMC workflow on [Proposal] Remove Scala from StreamPark — design and migration plan #4408
• Preserve Console submit/cancel/deploy functional parity

Acceptance

• No .scala under streampark-flink-proxy (src/main and src/test)
• Module builds and tests pass
• Downstream modules compile (where applicable)

References

• Epic: [Proposal] Remove Scala from StreamPark — design and migration plan #4408
• Roadmap: [Roadmap] StreamPark 3.0 #4410
• JDK 11: [Proposal] Migrate StreamPark Console baseline from JDK 8 to JDK 11 #4409 / [Build] Raise StreamPark compile and CI baseline to JDK 11 #4417 ✅
• Dev framework removal: [Flink] Remove dev framework and connector wrappers #4432 ✅

Notes

Critical: preserve classloader cache keys and ChildFirstClassLoader behavior (see AGENTS.md).