Closes: #6677

Summary by cubic

Make axios response header types case-insensitive so both uppercase and lowercase common header keys work without TS errors. Addresses issue #6677 and aligns with AxiosHeaders behavior.

Description

• Summary of changes
  • Added CommonResponseHeaderKey = CommonResponseHeadersList | Lowercase<CommonResponseHeadersList>.
  • Updated RawCommonResponseHeaders to use CommonResponseHeaderKey.
  • Applied the change in both index.d.ts and index.d.cts.
  • Merged v1.x formatting updates (quote normalization, condensed signatures); no runtime changes.
• Reasoning
  • Match HTTP’s case-insensitive headers and fix TS access for keys like response.headers['content-type'].
• Additional context
  • Resolves Response headers are lowercased, types are not #6677.

Docs

• Clarify that common response headers can be accessed in either case (e.g., Content-Type or content-type).
• No migration needed.

Testing

• No tests added.
• Recommend dtslint/TS checks to validate both response.headers['Content-Type'] and response.headers['content-type'] are accepted.

^{Written for commit a2bc503. Summary will update on new commits.}