feat(connections): opaque plugin-specific extra fields for ConnectionParams

[fuleinist]

Follow-up to the remaining core-side work from #593 (comment) — generic support for plugin-owned connection fields, not just DynamoDB.

What

• extra: HashMap<String, String> on core ConnectionParams (Rust) + extra?: Record<string, string> (TS). Opaque to the host: persisted verbatim through save_connection/update_connection, forwarded to driver plugins as part of params, omitted from persisted JSON when empty (serde(default, skip_serializing_if) — legacy JSON deserializes to an empty map).
• New plugin slot connection-modal.extra_fields, rendered below the host/port section of the connection form. Context exposes driver, the current extra map, and setExtraField(key, value) so a plugin UI can edit its own fields (empty value clears the key).
• updateExtraField pure helper in src/utils/connections.ts with 8 unit tests (set / merge / immutability / clear-on-empty / blank-key / trim).
• Host API version 0.1.0 → 0.2.0 (HOST_API_VERSION, plugin-api API_VERSION + package.json) — the slot map is observable to plugin bundles.
• PLUGIN_GUIDE.md slot table updated.

Verification

• vitest: PASS (3490) FAIL (0) (incl. new helper tests)
• tsc --noEmit: clean
• eslint on all touched files: 0 errors
• cargo clippy --all-targets: clean, no new warnings in touched files
• plugin-api check:sync: OK
• rustfmt on touched files: no new diffs vs upstream style

Note on local Rust test execution: on this Windows box the compiled test binary fails at load with STATUS_ENTRYPOINT_NOT_FOUND from an old system comctl32.dll missing TaskDialogIndirect — reproduced identically on pristine upstream code, so it's a local-environment issue unrelated to this change; CI (Linux) runs the full Rust suite.

Companion PR

DynamoDB plugin side (consumes extra["region"]): TabularisDB/tabularis-dynamodb-plugin#59

[debba]

Reviewed and tested this locally on Linux.

Full run on the branch: vitest 3490 passed across 194 files, cargo test --lib 1089 passed / 0 failed / 4 ignored, tsc --noEmit clean, eslint clean on the touched files, and clippy quiet in models.rs and plugins/driver.rs. So the Rust suite you couldn't run on that Windows box is green here, nothing to worry about on that front.

I checked the plumbing instead of just reading the diff. params_for_persistence clones the map so it survives save and update, duplicate_connection keeps it, every plugin RPC ships the whole params struct so drivers get extra on each call, and the edit path spreads the stored params back into the form, so opening and re-saving a connection doesn't quietly drop plugin fields. Legacy connections.json files still load fine and stay byte identical while the map is empty.

To exercise the new slot for real I wrote a small stub UI extension against connection-modal.extra_fields and drove it on a live MySQL connection: prefill from extra, per key edits, clear on empty, and the key disappearing from the persisted JSON once the last entry is gone all behave exactly as documented.

Clean, self contained design. Keeping driver specifics out of the core schema is the right call. Thanks!