Python A2A: Expose `supported_protocol_bindings` as configurable parameter (#6098)

* Expose supported_protocol_bindings as configurable parameter on A2AAgent

Add supported_protocol_bindings parameter to A2AAgent.__init__() allowing
users to configure which A2A protocol bindings (JSONRPC, GRPC, HTTP+JSON)
the client prefers when connecting to remote agents.

- Defaults to ["JSONRPC"] matching current behavior
- Passes through to ClientConfig for transport negotiation
- Replaces 4 hardcoded references with the configurable value

Closes #6057

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Fix empty list falsy trap and add fallback path test coverage

- Use 'is not None' check instead of 'or' to preserve explicit empty list
- Add test verifying empty list is not silently replaced with defaults
- Add test verifying fallback path uses custom bindings

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Document known protocol binding values in docstring

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Use Literal union for protocol binding type hint

Provides IDE autocomplete for known values while keeping the type
open for custom bindings (Literal is str at runtime).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

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Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>