Extract flow-run state proposal orchestration

[desertaxle]

Related to #21932

This PR moves flow-run state proposal retries and response interpretation into dedicated sync and async proposer classes. Existing propose_state entry points remain compatibility facades, so callers retain current behavior while follow-up PRs migrate to the narrower request interface.

Details

• Binds one state-request operation instead of requiring a complete Prefect client.
• Keeps WAIT retries and server-authoritative ACCEPT, REJECT, paused-rejection, and ABORT handling behind the proposer interface.
• Shares response interpretation across sync and async implementations.
• Adds one contract suite covering both implementations with in-memory request adapters.

Follow-up #22946 uses this interface to order heartbeat admission with each state request.

Checklist

• This pull request references any related issue by including "closes <link to issue>"
  • The dependent heartbeat PR closes Zombie-detection automation false-positives on healthy completed flows #21932.
• If this is a complex change, a maintainer has confirmed the proposed approach on the linked issue.
• If this pull request adds or changes functionality, it includes tests or explains why tests are not needed.
• If this pull request changes user-facing behavior, it updates documentation or explains why documentation is not needed.
  • This refactors internal proposal ownership without changing user-facing behavior.
• If this pull request removes docs files, it includes redirect settings in mint.json.
• If this pull request adds functions or classes, it includes helpful docstrings.

[chatgpt-codex-connector]

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[codspeed-hq]

Merging this PR will not alter performance

✅ 8 untouched benchmarks
⏩ 1 skipped benchmark¹

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_{Comparing flow-run-state-proposals/foundation (582e5db) with main (50a2a27)}

Footnotes

1. 1 benchmark was skipped, so the baseline result was used instead. If it was deleted from the codebase, click here and archive it to remove it from the performance reports. ↩