Drop the deprecated Task proxy wire fields

[conradbzura]

Description

Once the worker proxy propagates on the context chain (#230), the Task.proxy and Task.proxy_id wire fields exist only as a deprecated compatibility path for dispatch across mixed minor versions. Remove them at the v1 boundary.

• Drop proxy and proxy_id from the Task message in wire.proto and reserve field numbers 5 and 6 so they are never reused.
• Stop populating them in Task.to_protobuf; stop reading them in Task.from_protobuf.
• Remove the worker-side fallback — the proxy is materialized only from the chain-borne __proxy__.
• Remove the new-client dual-write path, if Propagate the worker proxy as a wool.ContextVar #230 introduced one.
• Remove or migrate the tests that exercise the legacy fields.

After this, the proxy reaches a worker through one mechanism: the context chain.

Motivation

The dedicated fields duplicate what the context chain already carries. #230 keeps them only so a client can dispatch to a worker on a different minor version within the same major version. A major-version boundary lapses that guarantee, so the redundancy and the fallback shim can retire, leaving a single propagation path and a smaller Task schema.

Expected Outcome

• Task.proxy and Task.proxy_id are gone from wire.proto; field numbers 5 and 6 are reserved.
• Task.to_protobuf and Task.from_protobuf no longer read or write either field.
• The worker materializes the proxy only from the chain-borne __proxy__; the fallback read is gone.
• The new-client dual-write path is removed.
• Tests that covered the legacy fields are removed or migrated to the chain path.
• The suite stays green.