fix(mcp): reject missing task input responses (#2816)

When `tasks/update` receives no response under a paused task's current
input-request key, it now returns `invalidParams` naming the expected
key. Previously it returned success while discarding the answer and
leaving the task paused.

Regression coverage checks unexpected keys, nil and empty maps, and nil
responses using both message IDs and the task-ID fallback, and verifies
that rejected responses do not dispatch a continuation or change task
status.

Fixes #2761.

Validation:
- Confirmed the regression test fails before the fix.
- `cd go && go test ./core/internal/mcp -count=1`
- `make -C go lint`

Signed-off-by: Eitan Yarmush <eitan.yarmush@solo.io>