feat: support PR-specific base resolution in recce instance

[wcchang1115]

PR checklist

• Ensure you have added or ran the appropriate tests for your PR.
• DCO signed

What type of PR is this?
Feature

What this PR does / why we need it:
This PR enables the instance to pass its session_id to the base-session download endpoint, allowing the cloud backend to resolve isolated base artifacts when available.

Which issue(s) this PR fixes:

Special notes for your reviewer:

• Instance-side only — cloud backend changes are in recce-cloud-infra
• Fully backward compatible: session_id defaults to None, preserving existing shared-base behavior

Does this PR introduce a user-facing change?:

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
recce/state/cloud.py	65.75% <100.00%> (ø)
recce/util/recce_cloud.py	42.85% <100.00%> (+5.25%)	⬆️
tests/state/test_cloud.py	99.79% <100.00%> (+0.01%)	⬆️
tests/util/test_recce_cloud.py	99.35% <100.00%> (+0.17%)	⬆️

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