fix: hydrate approval resume via Flow._state, not the read-only property

[aleksUIX]

Summary

Fixes #67. POST /approvals/{id}/resume 500s after a successful decide. _resume_proposal_flow assigned flow.state = ProposalState(**snapshot). CrewAI 1.15 made Flow.state a read-only @property, so __setattr__ raises AttributeError: property 'state' of 'ProposalHandlingFlow' object has no setter. Decide still writes the approval; only resume blows up.

The property reads the _state PrivateAttr. Existing tests already hydrate that way (flow._state = state in test_proposal_error_taxonomy.py). Resume now does the same. In-place mutation of flow.state.proposal_id etc. is unchanged.

Empty snapshots use snapshot or {} so ProposalState(**None) cannot throw.

Fix

flow = ProposalHandlingFlow()
flow._state = ProposalState(**(snapshot or {}))

Out of scope (deliberate)

• Does not take fix: reject change requests for dealless orders #65 (deal_id or "", deal_id_required) or Rate card is writable but is not read by the quote, deal-booking or negotiation pricing paths #69 (rate card).
• Does not change decide, list, or the CrewAI Flow class itself.

Test plan

• service: resume after approve hydrates snapshot and returns accepted
• service: resume after reject returns rejected without assigning flow.state
• HTTP: authenticated POST /approvals/{id}/resume after decide returns 200 (resumed_from_approval)
• uv run --locked ruff check src/ and ruff format --check src/ tests/ clean

Closes #67