MiniMax-M3 exposes a single binary thinking knob (adaptive|disabled)
on its OpenAI-compatible endpoint at api.minimaxi.com, but our
provider unconditionally emits reasoning_effort — which M3 rejects.
Detect api.minimaxi.com as a separate wire shape alongside
api.deepseek.com and:

  - emit thinking.type=adaptive|disabled in buildRequest
  - omit reasoning_effort entirely (M3 has no level scale)
  - translate /effort auto to 'adaptive' (the M3 default, since M3
    ships with thinking on out of the box and 'auto' semantically
    means 'don't override the model default')
  - map legacy level names from other vendors so a stale /effort
    value still resolves to a valid M3 level:
      off        → disabled  (retired DeepSeek 'no thinking' — M3
                              actually supports 'thinking off')
      low/medium/high → adaptive
      xhigh/max  → disabled
  - in NormalizeEffort, surface a MiniMax-specific usage hint
    (auto|adaptive|disabled) when an unknown level is supplied

internal/config/effort.go mirrors the change: M3 entries now expose
/levels=[auto,adaptive,disabled] with default=adaptive. Tests cover
the wire shape (TestBuildRequestMiniMaxThinking) and the
config-layer remap (TestNormalizeEffortMiniMax,
TestNormalizeEffortMiniMaxRejectsGarbage,
TestEffectiveEffortMiniMax).

Refactor: the host-matching logic was duplicated across
internal/provider/openai/openai.go (private *BaseURL helpers) and
internal/config/effort.go (private *Entry wrappers). Extract the
shared primitive into internal/provider/openai/host.go as
matchesVendorHost, plus thin exported wrappers IsDeepSeek and
IsMiniMax. The *Entry wrappers in effort.go now just gate on the
openai kind and delegate. Adding a new gateway is now a one-line
change in host.go rather than a four-line change across two files.

Add internal/provider/openai/host_test.go pinning the matching
rule (canonical hostname exact match, plus any subdomain of the
apex) directly at the source.