feat(pipes): warn at Put/bootstrap/refresh when a pipe can't be precisely cache-tracked (dummy-bind EXPLAIN diagnostic) #406

Follow-up agreed in the #343 review thread (#343 (comment), continuing #343 (comment)): degraded cache tracking is currently only observable when a pipe first executes (debug logs + the wavehouse_pipe_dep_resolution_fallback_total / wavehouse_pipe_dep_unresolved_total metrics). An operator authoring a pipe that reads s3(), a cross-database table, or a non-local dictionary — or one that can't be analyzed at all — should hear about it when the pipe is created or loaded, not discover a 10-second TTL cap in production.

Agreed design (from the thread)

The invalidation path stays exactly as #343 shipped it — lazy, per bound query, because the dependency set is parameter-dependent. The diagnostic doesn't need real parameter values: a table function / cross-database read shows up in EXPLAIN QUERY TREE regardless of what's bound, so we can dummy-bind the pipe's SQL and run the same EXPLAIN purely as a warning check:

• PUT /v1/pipes/{name} returns 200 with a warnings array (synchronous, best-effort — requires ClickHouse reachable; no warning path blocks the save, pipes are never rejected).
• Bootstrap (WH_PIPES_DIR seed files) logs a warning per affected pipe at load.
• A schema refresh that newly degrades a pipe (e.g. a view it reads became unfoldable) logs it.
• A pipe whose EXPLAIN fails outright at dummy-bind time (a write/DDL pipe, a parameter used as a whole table name) gets a "can't be cache-tracked; falls back to database-version invalidation" warning the same way.

Notes

• Warning taxonomy should mirror the runtime tiers from feat(pipes): resolve table dependencies for cache invalidation #343: database-version fallback (EXPLAIN failed) vs TTL-capped (external read / unfoldable view / pruned set).
• Dummy-bind needs a value per declared/inline parameter; bug(pipes): a declared parameter with no default binds NULL, skipping the inline default and the documented 400 #400 (a declared parameter with no default binds NULL) overlaps with that machinery.
• Related: bug(pipes): mutation pipe results are cached and coalesced — the write silently drops on repeat calls #386 (mutation pipe results shouldn't be cached/coalesced at all).