fix: Round 2 review findings — async SSRF, error sanitization, constants

Security:
- Make validate_endpoint async with hass.async_add_executor_job for DNS resolution
- Use _RestrictedIPError instead of fragile string matching for IP check flow
- Add is_multicast/is_unspecified to SSRF IP restriction checks
- Remove resolved private IP from error messages (generic message)
- Remove raw endpoint from __init__.py error log
- Sanitize error messages in metrics: strip URLs, API keys, Gemini key patterns
- Truncate API error response bodies before logging (512 chars)
- Use generic error messages in Gemini exception handlers (no str(e) interpolation)
- Pass api_key as explicit APIClient constructor parameter (not from header)
- Add defensive validation: Gemini provider requires api_key at construction

Code quality:
- Add constants: DEFAULT_INSTANCE_NAME, MIN/MAX_CONTEXT_MESSAGES, MIN/MAX_HISTORY_SIZE
- Replace all hardcoded schema ranges with named constants
- Move datetime import to module level in history.py
- Remove unused full_history/full_history_available keys
- Chain socket.gaierror properly with raise...from

custom_components/ha_text_ai/__init__.py

@@ -263,9 +263,9 @@ async def async_setup_entry(hass: HomeAssistant, entry: ConfigEntry) -> bool:
         model = config.get(CONF_MODEL, get_default_model(api_provider))
         raw_endpoint = config.get(CONF_API_ENDPOINT, get_default_endpoint(api_provider))
         try:
-            endpoint = validate_endpoint(raw_endpoint)
+            endpoint = await validate_endpoint(hass, raw_endpoint)
         except ValueError as err:
-            _LOGGER.error("Invalid API endpoint %s: %s", raw_endpoint, err)
+            _LOGGER.error("Invalid API endpoint: %s", err)
             raise ConfigEntryNotReady(f"Invalid API endpoint: {err}") from err
         # API key can now be updated via options
         api_key = config.get(CONF_API_KEY, entry.data.get(CONF_API_KEY))
@@ -292,6 +292,7 @@ async def async_setup_entry(hass: HomeAssistant, entry: ConfigEntry) -> bool:
             api_provider=api_provider,
             model=model,
             api_timeout=api_timeout,
+            api_key=api_key,
         )
 
         coordinator = HATextAICoordinator(