feat(whaleflow): mark mock cancellation and budgets

[Hmbown]

Summary

• add a serializable budget_exceeded workflow status plus explicit execution markers for cancellation and budget exhaustion
• teach the crate-local mock executor to stop on cancellation/global leaf budget/per-leaf zero-step budget
• add focused mock executor tests for cancelled and budget-exhausted branch/leaf runs

Stewardship boundary

This is a narrow #2669 slice. It stays inside codewhale-whaleflow mock execution and does not expose workflow_run, spawn agents, apply worktrees, persist TraceStore writes, or add live runtime cancellation wiring.

Credits: thanks @AdityaVG13 for the WhaleFlow draft/runtime direction in #2482/#2486.

Verification

• cargo test -p codewhale-whaleflow --locked
• cargo clippy -p codewhale-whaleflow --all-targets --locked -- -D warnings
• cargo fmt --all -- --check
• ./scripts/release/check-versions.sh
• ./scripts/release/check-ohos-deps.sh
• git diff --check
• cargo check --workspace --all-features --locked

[gemini-code-assist]

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