refactor: unify transport dispatch + collapse normalize shims

Consolidate 4 per-transport lazy singleton helpers (_get_anthropic_transport,
_get_codex_transport, _get_chat_completions_transport, _get_bedrock_transport)
into one generic _get_transport(api_mode) with a shared dict cache.

Collapse the 65-line main normalize block (3 api_mode branches, each with
its own SimpleNamespace shim) into 7 lines: one _get_transport() call +
one _nr_to_assistant_message() shared shim. The shim extracts provider_data
fields (codex_reasoning_items, reasoning_details, call_id, response_item_id)
into the SimpleNamespace shape downstream code expects.

Wire chat_completions and bedrock_converse normalize through their transports
for the first time — these were previously falling into the raw
response.choices[0].message else branch.

Remove 8 dead codex adapter imports that have zero callers after PRs 1-6.

Transport lifecycle improvements:
- Eagerly warm transport cache at __init__ (surfaces import errors early)
- Invalidate transport cache on api_mode change (switch_model, fallback
  activation, fallback restore, transport recovery) — prevents stale
  transport after mid-session provider switch

run_agent.py: -32 net lines (11,988 -> 11,956).

PR 7 of the provider transport refactor.

agent/anthropic_adapter.py

@@ -1666,42 +1666,3 @@ def normalize_anthropic_response(
         ),
         finish_reason,
     )
-
-
-def normalize_anthropic_response_v2(
-    response,
-    strip_tool_prefix: bool = False,
-) -> "NormalizedResponse":
-    """Normalize Anthropic response to NormalizedResponse.
-
-    Wraps the existing normalize_anthropic_response() and maps its output
-    to the shared transport types.  This allows incremental migration —
-    one call site at a time — without changing the original function.
-    """
-    from agent.transports.types import NormalizedResponse, build_tool_call
-
-    assistant_msg, finish_reason = normalize_anthropic_response(response, strip_tool_prefix)
-
-    tool_calls = None
-    if assistant_msg.tool_calls:
-        tool_calls = [
-            build_tool_call(
-                id=tc.id,
-                name=tc.function.name,
-                arguments=tc.function.arguments,
-            )
-            for tc in assistant_msg.tool_calls
-        ]
-
-    provider_data = {}
-    if getattr(assistant_msg, "reasoning_details", None):
-        provider_data["reasoning_details"] = assistant_msg.reasoning_details
-
-    return NormalizedResponse(
-        content=assistant_msg.content,
-        tool_calls=tool_calls,
-        finish_reason=finish_reason,
-        reasoning=getattr(assistant_msg, "reasoning", None),
-        usage=None,  # Anthropic usage is on the raw response, not the normaliser
-        provider_data=provider_data or None,
-    )

agent/transports/anthropic.py

@@ -78,13 +78,34 @@ class AnthropicTransport(ProviderTransport):
     def normalize_response(self, response: Any, **kwargs) -> NormalizedResponse:
         """Normalize Anthropic response to NormalizedResponse.
 
-        kwargs:
-            strip_tool_prefix: bool — strip 'mcp_mcp_' prefixes from tool names.
+        Calls the adapter's v1 normalize and maps the (SimpleNamespace, finish_reason)
+        tuple to the shared NormalizedResponse type.
         """
-        from agent.anthropic_adapter import normalize_anthropic_response_v2
+        from agent.anthropic_adapter import normalize_anthropic_response
+        from agent.transports.types import build_tool_call
 
         strip_tool_prefix = kwargs.get("strip_tool_prefix", False)
-        return normalize_anthropic_response_v2(response, strip_tool_prefix=strip_tool_prefix)
+        assistant_msg, finish_reason = normalize_anthropic_response(response, strip_tool_prefix)
+
+        tool_calls = None
+        if assistant_msg.tool_calls:
+            tool_calls = [
+                build_tool_call(id=tc.id, name=tc.function.name, arguments=tc.function.arguments)
+                for tc in assistant_msg.tool_calls
+            ]
+
+        provider_data = {}
+        if getattr(assistant_msg, "reasoning_details", None):
+            provider_data["reasoning_details"] = assistant_msg.reasoning_details
+
+        return NormalizedResponse(
+            content=assistant_msg.content,
+            tool_calls=tool_calls,
+            finish_reason=finish_reason,
+            reasoning=getattr(assistant_msg, "reasoning", None),
+            usage=None,
+            provider_data=provider_data or None,
+        )
 
     def validate_response(self, response: Any) -> bool:
         """Check Anthropic response structure is valid.