Fix axios header type narrowing in GLM file upload (re-apply)

[kiro-agent]

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PR #3 was merged before the follow-up "Fix axios header type narrowing in GLM file upload" commit landed, so main still carries the broken line that blocks npm run dev:backend:

backend/proxy/adapters/glm.ts(224,7): error TS2322:
  Type 'string | number | true | string[] | AxiosHeaders' is not assignable to type 'string'.
  Type 'number' is not assignable to type 'string'.

In axios v1 response.headers is AxiosHeaders, so response.headers['content-type'] is typed as string | number | true | string[] | AxiosHeaders rather than string | undefined. Assigning that union to a string local fails strict type checking under ts-node.

This re-applies the one-line fix: narrow the value with a typeof === 'string' check, falling back to mime.lookup(filename) and the generic octet-stream default for non-string headers.

Test plan

• git checkout main, pull this branch, run npm run dev and confirm the backend starts without TS errors.
• Existing GLM file-upload code path unchanged for the common case where the server returns a string content-type header.