feat: add POST /v1/cancel/{command_id} endpoint

[mlpy0]

Summary

• Adds explicit POST /v1/cancel/{command_id} REST endpoint to cancel in-flight text and image generation commands by ID
• Previously, cancellation only worked via HTTP disconnect (client closes SSE connection, triggering anyio.get_cancelled_exc_class()). Non-streaming clients and external consumers had no way to cleanly cancel active generation
• The endpoint looks up the command in active generation queues, sends TaskCancelled to notify workers, and closes the sender stream. Returns 404 (OpenAI error format) if the command is not found or already completed

Changes

File	Change
src/exo/shared/types/api.py	Add CancelCommandResponse model
src/exo/master/api.py	Import, route registration, cancel_command handler
src/exo/master/tests/test_cancel_command.py	3 test cases (404, text cancel, image cancel)
docs/api.md	Document new endpoint + update summary table

Design Decisions

• Uses self._send() (not raw command_sender.send()) — respects API pause state during elections
• Uses raise HTTPException — feeds into exo's centralized OpenAI-style error handler
• Returns typed CancelCommandResponse — consistent with CreateInstanceResponse / DeleteInstanceResponse patterns
• sender.close() is idempotent so concurrent cancel requests for the same command are harmless

Test Plan

• test_cancel_nonexistent_command_returns_404 — verifies 404 with OpenAI error format
• test_cancel_active_text_generation — verifies 200, sender.close() called, TaskCancelled sent with correct cancelled_command_id
• test_cancel_active_image_generation — same verification for image queue
• basedpyright — 0 new errors
• ruff check — all checks passed
• pytest — 3/3 new tests pass, no regressions

[Evanev7]

looks good, just a tidy

[Evanev7]

Suggested change

	async def cancel_command(self, command_id: str) -> CancelCommandResponse:
	"""Cancel an active command by closing its stream and notifying workers."""
	cid = CommandId(command_id)
	async def cancel_command(self, command_id: CommandId) -> CancelCommandResponse:
	"""Cancel an active command by closing its stream and notifying workers."""

[Evanev7]

the CommandId's are just strings anyway, we don't need the extra step here

[Evanev7]

cool, test seems a little redundant imo but lgtm

[mlpy0]

What did you do @Evanev7

[Evanev7]

just rebased onto latest main to try and get it merged, though we're feature freezing now before our next release

[mlpy0]

just rebased onto latest main to try and get it merged, though we're feature freezing now before our next release

Sorry for confusion

[Evanev7]

no worries! it's nice to see regular contributors so let me know if i can do anything to keep things clear

[mlpy0]

no worries! it's nice to see regular contributors so let me know if i can do anything to keep things clear

Thank you 🙏🙏