fix(chat-completions): strip timestamp from messages before sending to strict providers

Per-message timestamp metadata injected by _apply_persist_user_message_override
leaks into the Chat Completions payload sent to the provider. Strict OpenAI-compatible
providers (e.g. Fireworks-backed endpoints like OpenCode Go 'glm-5.2', Mistral, Kimi)
reject this schema-foreign field with HTTP 400:

  Extra inputs are not permitted, field: 'messages[0].timestamp'

The ChatCompletionsTransport.convert_messages already strips known internal-only
fields (tool_name, _-prefixed scaffolding keys, codex_reasoning_items, etc.) — add
timestamp to that list.

Closes #47868

agent/transports/chat_completions.py

@@ -172,6 +172,7 @@ class ChatCompletionsTransport(ProviderTransport):
                 "codex_reasoning_items" in msg
                 or "codex_message_items" in msg
                 or "tool_name" in msg
+                or "timestamp" in msg  # #47868 — strict providers reject this
             ):
                 needs_sanitize = True
                 break
@@ -201,6 +202,7 @@ class ChatCompletionsTransport(ProviderTransport):
             msg.pop("codex_reasoning_items", None)
             msg.pop("codex_message_items", None)
             msg.pop("tool_name", None)
+            msg.pop("timestamp", None)  # #47868 — leak into strict providers
             # Drop all Hermes-internal scaffolding markers (``_``-prefixed).
             # OpenAI's message schema has no ``_``-prefixed fields, so this
             # is safe and future-proofs against new markers being added.