Transport: validate fixture at place_call time to avoid partial session state

[brettheap]

Summary

FixtureDrivenTransport.event_stream() parses and validates the fixture JSON
after the orchestrator has already transitioned the session to in_flight
and incremented attempt_count. A malformed fixture (invalid JSON, a missing
events array, or an event missing type/event_id/timestamp) therefore
fails mid-stream.

As of PR #1 the failure is clean — event_stream() raises ValueError and
run-one converts it to an error: line + exit code 2 — but the SQLite state
store is left with a partially-progressed session row and a consumed
attempt. For recoverable operator input (a typo'd fixture path/JSON) the run
is therefore not idempotent at the workflow level.

Proposed fix

Parse and structurally validate the fixture in FixtureDrivenTransport.place_call()
— before the orchestrator mutates any session state — so a malformed fixture is
rejected with no session row created and no attempt consumed. event_stream()
can then assume a well-formed fixture.

Context

• Source: Codex review of commit 5e15f5b (finding 5); deferred from PR Slice 1 — Mock Call, Mock CRM (feature 001) #1.
• Affected: src/opencloser/transport/mock.py — place_call() / event_stream().
• Severity: P2 — the error already surfaces cleanly; the gap is the leftover partial state.
• Likely Slice 2 scope, but small enough to land standalone.

[brettheap]

Resolved by Slice 2 T035/T036 (PR #4 — US5/US6 foundation).

Fix landed:

• src/opencloser/transport/mock.py — validate_fixture() raises MalformedFixtureError for invalid JSON, missing events array, or an event missing type/event_id/timestamp.
• src/opencloser/transport/base.py — added the side-effect-free pre_validate_fixture(fixture_id) hook to the CallTransport protocol.
• src/opencloser/core/orchestrator.py — calls pre_validate_fixture() BEFORE session-row creation, so a malformed fixture leaves no session row, no consumed attempt, and no CRM queue change (FR-019, FR-020).

Verified:

• Unit: tests/unit/test_transport_fixture_validation.py
• Integration: tests/integration/test_us5_malformed_fixture.py — invalid JSON, missing events array, event missing identity field, missing fixture file (SC-006).

See spec specs/002-mock-call-real-crm/spec.md §User Story 5 + FR-019/FR-020 and the demo runbook at specs/002-mock-call-real-crm/quickstart.md §11.