feat: add batch computer action endpoint (POST /computer/batch) (#144)

# Checklist

- [ ] A link to a related issue in our repository
- [ ] A description of the changes proposed in the pull request.
- [ ] @mentions of the person or team responsible for reviewing proposed
changes.

## Summary

Adds a `POST /computer/batch` endpoint that accepts an array of computer
actions and executes them sequentially while holding a single input
lock, eliminating per-action network round-trip latency.

**Key changes:**

- **New endpoint** `POST /computer/batch` — accepts a
`BatchComputerActionRequest` with an ordered `actions` array. Supported
action types: `click_mouse`, `move_mouse`, `type_text`, `press_key`,
`scroll`, `drag_mouse`, `set_cursor`, and a new `sleep` action (with
`duration_ms` for configurable delays between actions)
- **Action schemas are identical** to the existing individual endpoint
request bodies (via `$ref`), so there's nothing new for users to learn
per-action
- **Execution semantics**: actions run sequentially under a single
`inputMu` lock acquisition; stops on first error and reports the failing
action index in the error message (e.g. `"actions[2] (click_mouse):
coordinates must be non-negative"`)
- **Refactored existing handlers** — extracted core validation+execution
logic from 7 computer control handlers into private `do*` methods (e.g.
`doClickMouse`, `doMoveMouse`), making the public handlers thin
wrappers. This avoids mutex re-entrancy issues and enables clean reuse
from the batch handler
- **Error classification** — introduced `validationError` (→ 400) and
`executionError` (→ 500) sentinel types with `isValidationErr()` helper
for consistent HTTP status mapping in both individual and batch handlers

## Test plan

- [x] All existing unit tests pass (`go test ./...`)
- [x] `go vet ./...` clean
- [x] New unit tests for error sentinel types (`TestValidationError`,
`TestExecutionError`, `TestIsValidationErr_Nil`)
- [ ] Manual testing of batch endpoint with a sequence of actions (e.g.
move_mouse → sleep → click_mouse)

Made with [Cursor](https://cursor.com)


<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Medium Risk**
> Touches multiple input-control handlers and changes how errors are
classified and surfaced, so regressions could affect automation
reliability and client-visible status codes. The new batch execution
path also increases the impact of validation/execution bugs because it
holds the input lock across a whole sequence.
> 
> **Overview**
> Adds `POST /computer/batch`, allowing clients to submit an ordered
list of computer actions (mouse, keyboard, cursor, plus a bounded
`sleep`) that are executed sequentially while holding the existing
`inputMu` lock; execution stops on the first failing action and returns
an error message annotated with the action index/type.
> 
> Refactors existing computer-control endpoints (`MoveMouse`,
`ClickMouse`, `TypeText`, `PressKey`, `Scroll`, `DragMouse`,
`SetCursor`) to delegate to shared `do*` helpers and introduces
`validationError` vs `executionError` to consistently map failures to
HTTP `400` vs `500` across both single-action and batch flows; updates
generated `oapi` types/clients, Swagger spec, and adds unit tests for
the new error classification helpers.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
8fdeda8. This will update automatically
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<!-- /CURSOR_SUMMARY -->

---------

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: rgarcia