feat(providers): add reasoning_echo_policy field for OpenAI-compat reasoning_content handling (#90)

Refs librefang/librefang#4842 — long-term replacement for the substring
match that the OpenAI driver currently uses to decide how to handle
`reasoning_content` on historical assistant turns.

Three provider-specific behaviours that the driver must distinguish at
wire time, now expressed as catalog metadata:

* `strip`  — DeepSeek R1 / deepseek-reasoner. The API rejects requests
  that carry reasoning_content on previous assistant messages.
* `echo`   — DeepSeek V4 Flash. Thinking mode is on by default and the
  API rejects multi-turn requests when assistant turns containing
  tool_calls don't echo back the original reasoning text. This is the
  bug surfaced in librefang/librefang#4842.
* `empty_string` — Moonshot / Kimi K2 family. The field must be present
  (empty string) on tool_calls turns, with thinking disabled wire-side
  for multi-turn compatibility.
* `none` (default) — most providers; field is omitted entirely.

V4 Pro is intentionally NOT marked `echo` — librefang#4842 reports it
working out-of-the-box; flip when there's an empirical reproducer.

Marks affected models:

  providers/deepseek.toml
    deepseek-v4-flash → echo
    deepseek-reasoner → strip
  providers/moonshot.toml
    kimi-k2.6, kimi-k2.5, kimi-k2 → empty_string
  providers/kimi-coding.toml
    kimi-for-coding → empty_string
  providers/byteplus-coding.toml
    kimi-k2.5 → empty_string
  providers/novita.toml
    moonshotai/kimi-k2-thinking → empty_string

Tooling:

* schema.toml registers the field with the four enum options and a
  `none` default so existing TOML files keep parsing unchanged.
* scripts/validate.py rejects unknown enum values; verified with a
  hand-crafted negative case (`reasoning_echo_policy = "bogus"` →
  validation fails with the expected message).
* `python3 scripts/validate.py` passes (267 models).

The librefang side that consumes this field will land in a follow-up
PR — until then, registry consumers ignore the field via
`#[serde(default)]` and the existing substring fallback continues to
work, so this commit is safe to ship independently.

schema.toml

@@ -152,6 +152,13 @@ required = false
 description = "Extended thinking / reasoning support"
 default = false
 
+[provider.sections.models.fields.reasoning_echo_policy]
+type = "string"
+required = false
+description = "How the OpenAI-compatible driver must handle the reasoning_content field on historical assistant turns when this model is used. 'none' (default) omits the field entirely. 'strip' is required by DeepSeek-R1 / deepseek-reasoner — the API rejects multi-turn requests that carry reasoning_content from previous turns. 'echo' is required by DeepSeek V4 Flash and other thinking-mode-on models — the original thinking text MUST be round-tripped on assistant turns that contain tool_calls, otherwise the API returns 400. 'empty_string' is required by Moonshot / Kimi K2 — the field must be present (empty string) on tool_calls turns, with thinking disabled wire-side."
+options = ["none", "strip", "echo", "empty_string"]
+default = "none"
+
 [provider.sections.models.fields.aliases]
 type = "array"
 required = false