fix(model_metadata): drop duplicate 'stepfun' entry in _PROVIDER_PREFIXES

[waroffchange]

Problem

"stepfun" is listed twice inside the same frozenset literal in agent/model_metadata.py:

• Line 50 — the canonical provider block, alongside gemini, zai, minimax, anthropic, deepseek.
• Line 63 — the # Common aliases block, alongside opencode, zen, go, kilo, dashscope.

stepfun is a canonical provider name, not an alias, so the second copy is both redundant and filed under the wrong heading. frozenset dedupes on construction, so the alias-block entry has never had any effect — it is dead weight that misleads the next reader into thinking stepfun is an alias of something.

Fix

Removes the duplicate from the alias block. One line, one file.

     "ollama",
-    "stepfun", "opencode", "zen", "go", "kilo", "dashscope", "aliyun", "qwen",
+    "opencode", "zen", "go", "kilo", "dashscope", "aliyun", "qwen",
     "mimo", "xiaomi-mimo",

Why this is behavior-preserving

_PROVIDER_PREFIXES is only ever consumed as a membership test, so a duplicate element is a strict no-op and removing it cannot change any outcome:

• "stepfun" in _PROVIDER_PREFIXES is still True — line 50 still provides it.
• The set's contents, size, and every lookup against it are unchanged.

No logic, no control flow, and no public surface is touched. The unrelated api.stepfun.ai / api.stepfun.com → stepfun endpoint-host map further down the file is deliberately left alone.

Verification

Provable by reading the literal — the same string appears twice in one frozenset({...}). Confirmed mechanically with an AST scan of the file (ast.Set element counting), which reports the frozenset going from 2 stepfun entries to 1 while the file's other two occurrences (the endpoint map) stay untouched.

[tonydwb]

Code Review Summary

Verdict: Approved

Drops duplicate 'stepfun' entry in _PROVIDER_PREFIXES. Trivial cleanup. No security concerns.

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Reviewed by Hermes Agent