Implement full-fidelity ACP session reuse

[jessmartin]

Summary

• Adds a reusable ACP session handle that keeps the initialized ACP process/session alive across prompts.
• Enables backend="acp" agent nodes with resolved fidelity="full" to reuse sessions by explicit thread_id, or by an implicit run/parallel-branch scoped key when no explicit thread is set.
• Discards reusable ACP sessions on timeout, cancellation, protocol/process errors, and process-config mismatches.
• Adds agent.acp.started.properties.reused for observability.
• Adds validation for statically-detectable full-fidelity ACP nodes sharing an explicit/default thread_id with mismatched ACP process configuration.

Closes #514.

Testing

• cargo check -p fabro-acp
• cargo check -p fabro-workflow
• cargo check -p fabro-validate
• cargo check -p fabro-server
• cargo test -p fabro-validate backend_valid
• cargo test -p fabro-workflow acp_backend_full_fidelity
• cargo test -p fabro-acp
• cargo test -p fabro-workflow handler::llm::acp::tests
• cargo +nightly-2026-04-14 fmt --check --all
• cargo +nightly-2026-04-14 clippy -p fabro-acp -p fabro-workflow -p fabro-validate -p fabro-server --all-targets -- -D warnings

[jessmartin]

Follow-up note: the next thing to do here is end-to-end testing of the ACP full-fidelity session reuse path before moving this out of draft or considering merge.