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.

tests/run_agent/test_run_agent_multimodal_prologue.py

@@ -13,7 +13,8 @@ They do NOT boot the full AIAgent — the prologue-fix guarantees are pure
 function contracts at module scope.
 """
 
-from run_agent import _chat_content_to_responses_parts, _summarize_user_message_for_log
+from run_agent import _summarize_user_message_for_log
+from agent.codex_responses_adapter import _chat_content_to_responses_parts
 
 
 class TestSummarizeUserMessageForLog: