feat(openai): add GPT-5.4 and GPT-5.4-mini (#67)

Add the two GPT-5.4 variants exposed by OpenAI's API (source:
https://developers.openai.com/api/docs/models/gpt-5.4 and
https://developers.openai.com/api/docs/models/gpt-5.4-mini):

- **gpt-5.4** — frontier tier, 1,050,000 context window, 128k max output,
  $2.50 / input MTok, $15.00 / output MTok.
- **gpt-5.4-mini** — balanced tier (matching the naming convention used
  by `gpt-5-mini`, `gpt-4.1-mini`, etc.), 400,000 context window,
  128k max output, $0.75 / input MTok, $4.50 / output MTok.

Ordered right after the GPT-5.2 family and before the Codex variants
section to keep the frontier-GPT chain in release order.

Addresses librefang/librefang-registry#65 — the original request filed
these under the `codex-cli` provider, but Codex CLI's upstream
`models.json` doesn't list `gpt-5.4-mini` (only the full `gpt-5.4`
slug is list-visible there, and it's already tracked in
`providers/codex-cli.toml`). The correct home for OpenAI-API-direct
access is this file; users who want `gpt-5.4-mini` should configure
`provider = "openai"` rather than `provider = "codex-cli"`.

providers/openai.toml

@@ -1,5 +1,5 @@
 # OpenAI — https://openai.com
-# Models: 13 (including 2 Codex variants)
+# Models: 15 (including 2 Codex variants)
 
 [provider]
 id = "openai"
@@ -154,6 +154,32 @@ supports_vision = true
 supports_streaming = true
 aliases = []
 
+[[models]]
+id = "gpt-5.4"
+display_name = "GPT-5.4"
+tier = "frontier"
+context_window = 1050000
+max_output_tokens = 128000
+input_cost_per_m = 2.50
+output_cost_per_m = 15.0
+supports_tools = true
+supports_vision = true
+supports_streaming = true
+aliases = []
+
+[[models]]
+id = "gpt-5.4-mini"
+display_name = "GPT-5.4 Mini"
+tier = "balanced"
+context_window = 400000
+max_output_tokens = 128000
+input_cost_per_m = 0.75
+output_cost_per_m = 4.50
+supports_tools = true
+supports_vision = true
+supports_streaming = true
+aliases = []
+
 # Codex variants (same OpenAI driver & API key)
 
 [[models]]