feat: add execution progress handler

[peterfoldes]

Summary

Add Connection.set_progress_handler() that lets callers receive real-time execution progress events from the SQL session.

When a handler is registered, execute_sql requests automatically include enable_progress_events: true. The server streams execution_progress events with task counts, and the SDK dispatches them to the user's callback. Pass None to disable.

This is a backwards-compatible change. The SDK can safely send enable_progress_events: true to older server versions that don't support it — the unknown field is silently ignored and queries execute normally. Progress events are best-effort: depending on the server version and query type, the handler may not be invoked.

Companion server-side PR: https://github.com/wherobots/sql-session/pull/189

Related Issues

Relates to sql-session#189

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Requester Checklist

Complete these before marking Ready for Review

• I have self-reviewed my own code
• I have added/updated tests that prove my fix/feature works
• I have included visual proof (screenshot, video, or test output) if applicable
• All CI checks are passing
• PR size is S/M, OR I have justified the size and added a walkthrough
• I have updated documentation if needed

Visual Proof

Tested end-to-end on staging with a custom image built from the sql-session peter/execution-progress branch:

Without --progress (baseline — no regression):

Request: {"kind": "execute_sql", "execution_id": "dfe6b4eb-...", "statement": "SELECT 1 AS test_col"}
┏━━━┳━━━━━━━━━━┓
┃ # ┃ test_col ┃
┡━━━╇━━━━━━━━━━┩
│ 0 │ 1        │
└───┴──────────┘

With --progress (progress events flowing):

Request: {"kind": "execute_sql", ..., "enable_progress_events": true}
< {"kind":"execution_progress","execution_id":"90199781-...","tasks_total":0,"tasks_completed":0,"tasks_active":0}
  [progress] ?  0/0 tasks (0 active)  90199781
┏━━━┳━━━━━━━━━━┓
┃ # ┃ cnt      ┃
┡━━━╇━━━━━━━━━━┩
│ 0 │ 10000000 │
└───┴──────────┘

Backwards compatibility (against current released staging, no custom image):

Request: {"kind": "execute_sql", ..., "enable_progress_events": true}
(no progress events received — server ignores unknown field)
┏━━━┳━━━━━━━━━━┓
┃ # ┃ test_col ┃
┡━━━╇━━━━━━━━━━┩
│ 0 │ 1        │
└───┴──────────┘

Size Justification (if L/XL)

N/A — small PR (+83/-2 lines across 4 files)

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Reviewer Checklist

If these are not met, close the tab — this PR is not ready for review

• Requester checklist above is complete
• All CI checks are passing
• Tests adequately cover the changes