This PR adds a `try? => tac` syntax that runs `evalSuggest` directly on
a given tactic, useful for testing the `try?` machinery in isolation. It
also adds a server_interactive test (`cancellation_par.lean`) that
demonstrates a cancellation bug with parallel tactic combinators.
The test contrasts three combinators:
- **`first`** (sequential): cancellation works correctly — the tactic
runs on the main elaboration thread and shares its cancel token.
- **`attempt_all_par`** (parallel): cancellation is broken — the subtask
spawned via `asTask` gets a fresh cancel token that is never set on
re-elaboration.
- **`first_par`** (parallel): same bug as `attempt_all_par`.
The test uses a `check_cancel <label>` helper tactic that detects leaked
cancel tokens without any timing dependency: the second invocation (from
re-elaboration) signals the first, which then checks whether its cancel
token was set.
Related issue: #13300
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Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
