fix: use POST for SubscribeToTask binding

[jstar0]

Summary

• Change the SubscribeToTask HTTP annotation from GET /tasks/{id}:subscribe to POST /tasks/{id}:subscribe.
• Apply the same change to the tenant-scoped binding.
• Add body: "*" to both custom POST bindings so the proto passes the API linter.

Changes

• Updated the default SubscribeToTask REST binding to use post.
• Updated the tenant-scoped SubscribeToTask REST binding to use post.
• Added body: "*" to both custom POST bindings, matching the API linter requirement for custom POST methods.

Why

The protocol documentation already lists Subscribe to task as POST /tasks/{id}:subscribe in the REST method mapping and task operations sections. The proto annotation still used get, so generated REST bindings could disagree with the published specification.

Scope

This only updates the SubscribeToTask HTTP binding in specification/a2a.proto. It does not change the request message shape or any other task operation.

Fixes #1819
Fixes: #1574

Verification

(cd specification && buf dep update && buf export buf.build/googleapis/googleapis --output=.googleapis)
api-linter --config specification/.api-linter.yaml --output-format github --set-exit-status --proto-path specification --proto-path specification/.googleapis specification/a2a.proto
protoc -I specification -I specification/.googleapis --descriptor_set_out=/tmp/a2a.pb specification/a2a.proto
./scripts/build_docs.sh

[gemini-code-assist]

Code Review

This pull request updates the SubscribeToTask RPC in specification/a2a.proto by changing its HTTP method from GET to POST and adding a wildcard body mapping (body: "*") for both the primary and additional bindings. There are no review comments, and I have no feedback to provide.

[darrelmiller]

Action: Review all SDKs to see if any actually use GET. If we can convince ourselves that it doesn't break any existing deployments, we should accept this.

[muscariello]

@darrelmiller @herczyn

JS/Go/Java use POST now
Python/Rust use POST as canonical method while GET as legacy.

[Tehsmash]

Going backwards in the repo this was a bug ('POST' in specification and 'GET' in the proto) since the methods were introduced in the specification in v0.2.6, we just never noticed:

4cfa452#diff-eebad86dc55e949d1d0630c4a2e92fed2a16b19e0ab623719ed429b5d3eee101R1084

A2A/specification/grpc/a2a.proto

Line 71 in 4cfa452

get: "/v1/{name=tasks/*}:subscribe"

As all the SDKs seem to have standardised on the written spec 'POST', for v1.0 I think we correct this to 'POST', for v0.3 there isn't much we can do.