Answer 404 when revoking a function from a component that does not exist

[asasemahmed]

What this changes

DELETE /api/components/:name/functions/:function calls store.revokeFunction, which was the only
one of the four component grant writes that did not ask whether the component exists: grant,
revoke and grantFunction all call requireComponent first, and the routes for the other three
catch ComponentNotFoundError and answer 404. revokeFunction just deleted, so against a name
nobody has it deleted zero rows, the route answered { "revoked": true }, and component.function_revoked
was written for a component that was never there.

revokeFunction now calls requireComponent and the route answers 404 with the store's own message,
as POST /:name/functions does.

The principle #616 records, that taking something away is always allowed so a dead row stays
removable by hand, does not apply here. component_functions.component_name references
components.name with ON DELETE CASCADE, so a function grant cannot outlive its component and a
revoke against a missing one can only ever delete nothing.

Where it runs

• New state that outlives a request? None.
• What happens on the second replica? The same: one extra read of components before the delete.
• Anything serialised? No. A component removed between the check and the delete leaves nothing to delete, as before.
• Anything fanned out to a browser? No.
• New listener, port, or schedule? No.

Boundary and audit

• Every acting call still goes through the gateway: grants are not acting calls, and how a granted function is used is unchanged.
• New refusals and new failures each write a row: the 404 writes none, like the 404s on the sibling routes. What changes is that a row is no longer written for something that did not happen.
• Nothing new is trusted from the client.

Changelog

• An entry in CHANGELOG.md under Unreleased.

Proof

• component-grants-skills-validation.test.ts: a revoke against a component that does not exist
  answers 404 with "No component is called nothing." and records no audit row; against one that
  does, it revokes, answers { revoked: true } and records component.function_revoked.
• component-store.integration.test.ts, beside the existing "withholding a component that does not
  exist is an error, not a silent row": revoking a function from a missing component throws
  ComponentNotFoundError.
• Linux, against Postgres: those two files and sandboxed-components.integration.test.ts, 51 pass.
  Against main, the two new cases fail.
• bun run format:check, bun run lint, bun run typecheck: clean.