crashdb_impl: improve GitHub API robustness

[Arnab-atra]

Summary

This change improves the robustness of the GitHub crash database implementation.

Changes

• Add appropriate Content-Type headers.
• Request JSON responses for OAuth endpoints.
• Improve type annotations using Mapping[str, Any].
• Decode responses before parsing JSON.
• Preserve tracebacks by using raise.
• Handle missing OAuth interval values safely.
• Make repository and label handling more robust.

[codecov]

Codecov Report

❌ Patch coverage is 22.85714% with 27 lines in your changes missing coverage. Please review.
✅ Project coverage is 29.96%. Comparing base (d9d3c9f) to head (3236b15).

Files with missing lines	Patch %	Lines
tests/integration/test_github.py	0.00%	22 Missing ⚠️
apport/crashdb_impl/github.py	61.53%	5 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (d9d3c9f) and HEAD (3236b15). Click for more details.

HEAD has 2 uploads less than BASE

Flag	BASE (d9d3c9f)	HEAD (3236b15)
	3	1

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #637       +/-   ##
===========================================
- Coverage   84.47%   29.96%   -54.51%     
===========================================
  Files         106       90       -16     
  Lines       21053    19778     -1275     
  Branches     3223     2956      -267     
===========================================
- Hits        17784     5926    -11858     
- Misses       2792    13348    +10556     
- Partials      477      504       +27     

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