[Feat]: contexts APIs needed

### Is your feature request related to a problem? Please describe.

The A2A protocol is "agent to agent", not "human to agent". But, if you're building a chat experience, then you'll want to use A2A (or perhaps "H2A" Human-2-Agent!) to to be able to list and search contexts to find your chats

Perhaps this should be an extension?

### Describe the solution you'd like

```protobuf
message Context {
string id = 1 [(google.api.field_behavior) = REQUIRED];
}

// List tasks with optional filtering and pagination.
rpc ListContexts(ListContextsRequest) returns (ListContextResponse) {
option (google.api.http) = {
get: "/contexts"
additional_bindings: {
get: "/{tenant}/contexts"
}
};
}

// List tasks with optional filtering and pagination.
rpc SearchContexts(SearchContextsRequest) returns (SearchContextResponse) {
option (google.api.http) = {
post: "/contexts/search"
additional_bindings: {
get: "/{tenant}/contexts"
}
};
}

```

### Describe alternatives you've considered

_No response_

### Additional context

_No response_

### Code of Conduct

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