## What
Adds label filtering to the `search_transactions` MCP tool. It's the one
filter the web transactions view has that the MCP surface was missing.
- New optional `label_ids` array param. Returns transactions carrying
**any** of the given labels (OR-semantics), matching the web app's label
filter (`Transaction::scopeApplyFilters`).
- Combines with the other filters (account, category, date, amount) with
AND, consistent with how the tool already treats them — so `category_id`
+ `label_ids` means "in that category **and** carrying one of these
labels".
- Each returned transaction now also exposes its `labels` (`id`,
`name`), so the agent can see why a row matched, consistent with what
the write tools already return.
## Validation & consistency
- `label_ids` are validated against the space via `labelsInSpace`,
promoted from `WriteTool` to the shared `McpTool` base so read and write
tools use one implementation. An unknown or stale label id now returns
the same actionable *"call list_labels"* error the rest of the MCP
surface gives, instead of a silent empty result.
- The tool `#[Description]` now mentions `label`, so agents discover the
capability without inspecting each param.
## Tests
- `search_transactions` filters by label id (matching row in, unlabelled
row out).
- A foreign/unknown label id is rejected with an error.
- Full MCP read + write suites stay green (26 tests) — confirms moving
`labelsInSpace` to the base didn't regress the write tools.
Pint, Larastan and the affected Pest suites pass locally.
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