fix(external): fix Content-Disposition header parsing for filename extraction

[pinglin]

Because

• The Content-Disposition header parsing logic had an incorrect condition that prevented filename extraction from HTTP responses
• The original code checked disposition == "" but then tried to access the disposition string, causing filenames to never be extracted from HTTP headers
• This affected file uploads where the filename should be preserved from the original HTTP response
• The base64 encoding issue for uploaded filenames was related to this parsing problem

This commit

• Fixes the Content-Disposition header parsing logic by changing the condition from disposition == "" to disposition != ""
• Removes the unnecessary check for "attachment" prefix, allowing filename extraction from both "attachment" and "inline" dispositions according to RFC standards
• Adds comprehensive test coverage for filename extraction from various Content-Disposition header formats including:
  • Basic attachment and inline dispositions
  • RFC 5987 encoded filenames with special characters
  • Complex filenames with quotes and special characters
  • Data URI parsing with filename parameters
  • URL-encoded filename handling
• Adds tests for both binaryFetcher and artifactBinaryFetcher implementations

---

Note

Fixes filename extraction from HTTP responses and adds extensive tests for URL/data URI handling and MinIO presigned patterns.

• Bug Fix
  • Correct Content-Disposition parsing in pkg/external/external.go to extract filename when header is present (handles both attachment and inline).
• Tests (pkg/external/external_test.go)
  • Add coverage for BinaryFetcher and ArtifactBinaryFetcher:
    • Filename extraction from Content-Disposition (attachment, inline, RFC 5987, complex names, absent header).
    • Data URI parsing with optional filename parameter and error cases.
    • Presigned URL decoding via v1alpha/blob-urls/{base64} and deprecated pattern matching.
    • Regular URL fallback and network/error handling.

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[linear]

INS-8668 [Bug] Fix base64 encoding issue for uploaded filename in backend