test(e2e): pin the a2a bridge agent's Anthropic key so message/send works (#34512)

The a2a completion-bridge tests registered the agent with only
custom_llm_provider and model, so the bridge's litellm.acompletion had no
api_key and relied on the gateway resolving ANTHROPIC_API_KEY from its ambient
env. When that env var is absent, POST /a2a/{id} returns 500 with
"Missing Anthropic API Key" and every message/send test (completion bridge,
pinned v0.3/v1.0 message shapes, semver serves) fails while the
register/discovery/rejection tests still pass.

Give A2ABridgeParams an optional api_key and register the bridge agent with
api_key="os.environ/ANTHROPIC_API_KEY", matching how the rest of the suite
wires anthropic-backed models (e.g. the ratelimit redis tests). The agent now
carries its provider key explicitly instead of depending on ambient gateway env.

tests/e2e/a2a/a2a_client.py

@@ -85,6 +85,7 @@ class A2ABridgeParams(BaseModel):
 
     custom_llm_provider: str
     model: str
+    api_key: str | None = None
 
 
 class AgentRegisterBody(BaseModel):

tests/e2e/a2a/test_a2a_agent_e2e.py

@@ -32,7 +32,11 @@ from e2e_config import unique_marker
 from e2e_http import Result, UnknownApiError, unwrap
 from lifecycle import ResourceManager
 
-BRIDGE = A2ABridgeParams(custom_llm_provider="anthropic", model="claude-haiku-4-5")
+BRIDGE = A2ABridgeParams(
+    custom_llm_provider="anthropic",
+    model="claude-haiku-4-5",
+    api_key="os.environ/ANTHROPIC_API_KEY",
+)
 
 MOVEHOME_AGENT_CARD_URL = "https://movehome.org/.well-known/agent.json"
 MOVEHOME_ORIGIN = "https://movehome.org"