fix(providers): align GPT-5.6 with the GA API surface

- every 5.6 tier accepts effort "max" and reasoning.mode
  "standard"/"pro" (GA docs: pro is a request mode on any GPT-5.6
  model) -- drop the Sol-only gating
- GPT-5.6 deprecates prompt_cache_retention; send
  prompt_cache_options={"ttl": "30m"} (its only supported lifetime)
  and keep the 24h retention policy for pre-5.6 models
- never inject commercial cache params into local lanes: dropped from
  the Chat Completions lane (which serves only openai-compatible and
  google) and gated off the compat-pinned Responses lane -- a gpt-5*
  served-model name is not an OpenAI account
- account cache writes: usage *_tokens_details.cache_write_tokens
  flows into cache_creation_tokens (5.6 bills writes at 1.25x the
  uncached input rate)
- drop non-string verbosity/reasoning_mode overrides with a warning
  instead of raising on unhashable capability-JSON values
- keep ModelCapabilities' public positional prefix stable by appending
  the verbosity/pro fields at the tail; pin it with a constructor test
- openai floor 2.44 -> 2.45, the first release with the typed
  prompt_cache_options kwarg

docs/api-reference.md

@@ -458,7 +458,7 @@ Each item in `items` (shared by `tool_info` and `approve_request`):
 | `context_window`         | int    | Total context window size in tokens                  |
 | `pct`                    | float  | Percentage of context window used                    |
 | `effort`                 | string | Reasoning effort level (`low`/`medium`/`high`)       |
-| `cache_creation_tokens`  | int    | Tokens written to prompt cache (Anthropic)           |
+| `cache_creation_tokens`  | int    | Tokens written to prompt cache (Anthropic + OpenAI)  |
 | `cache_read_tokens`      | int    | Tokens served from prompt cache (Anthropic + OpenAI) |
 
 **`info`** -- an informational message (e.g. command output).