fix(compat): remove ListTasks from v0.3 REST adapter (a2aproject#1043) (

a2aproject#1068)

## Summary

Fixes a2aproject#1043.

The A2A v0.3 spec does not define a `ListTasks` method, but
`REST03Adapter` advertised `GET /v1/tasks` while
`REST03Handler.list_tasks` raised `NotImplementedError` — exposing a
route guaranteed to fail at runtime.

This PR aligns the v0.3 REST adapter with the rest of the v0.3 compat
surface:
- **Removes** `('/v1/tasks', 'GET')` from `REST03Adapter.routes()`.
- **Removes** the dead `REST03Handler.list_tasks` stub.
- **Replaces** the stale `pytest.raises(NotImplementedError)` test with
a regression test asserting the route is no longer registered in
`REST03Adapter.routes()`.

This matches the design already present in the three sibling v0.3
transports — `jsonrpc_transport.py`, `grpc_transport.py`, and
`rest_transport.py` — all of which explicitly raise
`NotImplementedError('ListTasks is not supported in A2A v0.3 ...')`.
(Confirmed with @ishymko in the issue thread to go with Option 2 from
the report.)

## Known follow-up (out of scope)

With the route gone, `GET /v1/tasks` now falls through to the tenant
catch-all (`Mount('/{tenant}', ...)` in `rest_routes.py`). A request
with `A2A-Version: 0.3` will get a 400 version-mismatch error instead of
a clean 404/405. This is a pre-existing routing quirk that affects any
`/v1/<unknown>` path, not just `/v1/tasks` — worth a separate issue to
tighten the tenant path converter or register an explicit 405 for
reserved prefixes when v0.3 compat is enabled. Happy to follow up.

## Test plan

- [x] `uv run pytest tests/compat/v0_3/` — 249 passed
- [x] `./scripts/lint.sh` — ruff clean; `ty` reports only pre-existing
errors in untouched lines (generated protobuf stubs)
- [x] Manual trace: `GET /v1/tasks` no longer routes to
`REST03Handler.list_tasks`; the route is absent from
`REST03Adapter.routes()`

Co-authored-by: Iva Sokolaj <102302011+sokoliva@users.noreply.github.com>