feat: add handoff funnel telemetry

[captainsafia]

Summary

• Add client telemetry events for local-cloud and cloud-cloud handoff funnel stages, snapshot outcomes, spawned-without-snapshot, API/session outcomes, and hotswap attachment.
• Return structured local handoff snapshot upload outcomes so manifest failures, partial blob uploads, empty workspace skips, and token allocation failures are distinguishable.
• Update follow-up stream tests for the API-accepted event and add telemetry serialization coverage.

Validation

• cargo +1.92.0 test --manifest-path /workspace/warp/Cargo.toml -p warp --lib ai::ambient_agents::telemetry::tests:: passed (3 tests).
• cargo +1.92.0 test --manifest-path /workspace/warp/Cargo.toml -p warp --lib ai::ambient_agents::spawn::tests::followup passed (9 tests).
• rustfmt +1.92.0 --config-path /workspace/warp/.rustfmt.toml ... ran on touched Rust files.
• git -C /workspace/warp --no-pager diff --check passed.

Co-Authored-By: Oz oz-agent@warp.dev

Conversation: https://staging.warp.dev/conversation/bb5694d2-c4a9-4171-a5d4-9006c6a4308f
Run: https://oz.staging.warp.dev/runs/019e389e-7f5c-7d01-85af-a449c008e618
This PR was generated with Oz.