fix(servers): data-unavailable error for every DB tool, no DB names

[ShuxinLin]

Description

Follow-up to #554. The missing-database message from #554 was hidden in IoT/FMSR by older if not db checks, and several other servers never used it. I audited every MCP tool backed by CouchDB (85 tools, 6 servers). In each case the tool was run against (a) a CouchDB with no databases, (b) an unreachable CouchDB, and (c) databases that exist but are empty. The test databases had sentinel names, so any leaked name would show up.

Every DB-touching tool now returns the shared message when its database is missing or unreachable:

the data source does not exist or is unreachable in this environment; the data is unavailable, do not retry with other arguments

No error contains a database name, host, port or URL. Wrong-key messages (unknown site, No sensors found…, model 'x' not found, …) are unchanged when the database exists.

Fix Details

• servers/db_errors.py (new): DATA_UNAVAILABLE, plus db_failure_text(exc), which replaces requests/httpx exception text (it embeds the URL, and so the DB name) with database request failed (<ExcType>).
• iot: couchdb3.Database.__bool__ sends a HEAD request, so if not iot_db fired before the missing-DB path ("IoT records database not connected"), and it raised ConnectionError with the URL when CouchDB was down. The tool guards now use _missing_db_error, and helpers check is None. unknown site is checked against a missing registry first, because the registry falls back to ["MAIN"]. sites() reports a missing registry instead of returning ["MAIN"].
• fmsr: same if not fm_db issue. Lookup failures no longer include client exception text.
• wo: a connection failure raised "All connection attempts failed" and now raises DatabaseUnavailable. HTTP errors no longer embed the request URL (database request failed (HTTP 500)). get_failure_codes returns the shared message when the database is unavailable. next_wonum now retries only on CouchConflict (409), so HTTP errors aren't reported as counter contention.
• vibration: if not db raised with the host when CouchDB was down. It now checks is None.
• utilities: the catalog tools returned CouchDB's raw error body when the DB was missing, and a connection error naming the host when it was down. Both now return the shared message.
• tsfm: CouchStore treated a 404 for a missing database as a missing document. A catalog that was never loaded therefore read as empty ("models": [], model 'x' not found), and with CouchDB down every catalog and run tool leaked /<catalog_db>/_find plus the host. Requests now raise StoreUnavailable or StoreError without the URL. Model and feature catalog reads and registrations check that the catalog exists (Store.exists). Result and run collections are still created on first write, so a missing one still means "none yet".

Wire note: sites() now returns Union[SitesResult, ErrorResult], the same as every other IoT tool. Its text content is unchanged. Its structuredContent is now wrapped in {"result": …} like the other tools. No caller in the repo reads the flat shape.

Impact on Benchmarking

• No change to baselines: This fix only improves stability/performance.
• Baseline change: This fix corrects a scoring error. (Please provide "Before vs. After" results).

Related Issues

• Follow-up to fix: distinct error message when a CouchDB database is missing #554

Verification Steps

1. uv run pytest src/ -q -k "not integration": 696 passed. The 8 failures left (evaluation/test_static_json_scorer.py, observability/test_file_exporter.py) also fail on main.
2. New tests per server cover an unreachable CouchDB (a real couchdb3/httpx/requests client against a closed port) and a missing database, and assert the shared message with no DB name. On main, the IoT regression tests fail with IoT records database not connected / unknown site SITE_A.
3. Audit harness (not committed): a fake CouchDB plus a closed port, calling all 85 tools. Results: 0 leaks in any mode, and 0 false "unavailable" when the databases exist but are empty. The 24 tools that never touch CouchDB (DSP, time, local files, templates) are unaffected.

Checklist

• I have added tests that prove my fix is effective.
• My code follows the project's Ruff formatting and linting rules. (ruff isn't installed locally and CI doesn't run it; I checked line lengths by hand)
• I have signed off my commits (DCO).