feat(bitbucket): add server-level hook-script management methods

[mrcrysta1]

Summary

Adds the four remaining server-level hook-script endpoints for Bitbucket
Data Center (8+), completing the coverage requested in #1554. The
project/repository-level methods already existed; this PR closes the global
management gap:

• get_hook_script(script_id) → GET /rest/api/latest/hook-scripts/{scriptId}
• get_hook_script_content(script_id) → GET /rest/api/latest/hook-scripts/{scriptId}/content (raw body, uses not_json_response=True)
• update_hook_script(script_id, content, name, hook_type, description=None) → PUT /rest/api/latest/hook-scripts/{scriptId} (multipart form, mirrors create_hook_script)
• delete_hook_script(script_id) → DELETE /rest/api/latest/hook-scripts/{scriptId}

Each method follows the existing create_hook_script style: hook_type
validated against PRE/POST, multipart fields passed as tuples, and the
X-Atlassian-Token: no-check header for state-changing requests.

Changes

• atlassian/bitbucket/__init__.py: 4 new methods next to create_hook_script
• tests/test_bitbucket_server.py: 6 new tests in TestHookScripts (URLs,
  multipart fields, hook-type validation, not_json_response flag)
• docs/bitbucket.rst: usage example for the new server-level methods

Validation

• pytest tests/test_bitbucket_server.py -k HookScript: 9 passed
• black --check atlassian/bitbucket/__init__.py tests/test_bitbucket_server.py: clean
• pyflakes: clean

Notes

This work was developed with AI assistance (code generation and tests), and
reviewed by a human before submission.

[gonchik]

Thanks @mrcrysta1

[codecov]

Codecov Report

❌ Patch coverage is 92.30769% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 59.76%. Comparing base (6f49a1a) to head (c28e4c7).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
atlassian/bitbucket/__init__.py	92.30%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1653      +/-   ##
==========================================
+ Coverage   59.74%   59.76%   +0.02%     
==========================================
  Files         101      101              
  Lines       16868    16881      +13     
  Branches     1746     1748       +2     
==========================================
+ Hits        10077    10089      +12     
  Misses       6393     6393              
- Partials      398      399       +1     

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