fix(auth): set explicit User-Agent on Jira/Confluence sessions

[StefanWinter-ViREQ]

Summary

• Some WAFs in front of Jira/Confluence Server/DC reject the default python-requests/X.Y User-Agent and return 403, even when the bearer token is valid. The error surfaces in mcp-atlassian as a misleading Authentication failed for Jira API (403). Token may be expired or invalid — although the same PAT works via curl.
• JiraClient and ConfluenceClient now set User-Agent: mcp-atlassian/<version> on the session right after the underlying atlassian client is created, before _apply_custom_headers(), so user-supplied JIRA_CUSTOM_HEADERS=User-Agent=... / CONFLUENCE_CUSTOM_HEADERS=User-Agent=... still wins.
• Adds utils/user_agent.py::get_default_user_agent() (uses importlib.metadata.version("mcp-atlassian"), falls back to 0.0.0 when not installed — same pattern as the package __version__).

Reproduction (Jira DC 9.12.4 behind a WAF)

from atlassian import Jira
jira = Jira(url="https://portal.example.com", token=PAT, cloud=False)
jira.get_issue("PROJ-1")
# requests.exceptions.HTTPError: 403 Client Error  (default UA: python-requests/2.x)

# Workaround (now built-in by this PR):
jira._session.headers["User-Agent"] = "mcp-atlassian/0.21.1"
jira.get_issue("PROJ-1")   # 200 OK

Direct curl with the same URL + Bearer token consistently returns 200. The WAF blocks only the default python-requests/X.Y UA — any identifiable UA (curl/*, Mozilla/*, mcp-atlassian/*, …) is allowed.

Test plan

• uv run pytest tests/unit/jira/test_client.py tests/unit/confluence/test_client.py tests/unit/jira/test_custom_headers.py tests/unit/confluence/test_custom_headers.py — all green
• pre-commit run (ruff, ruff-format, mypy) — green
• Pre-existing failures on main (test_stdio_lifecycle, test_date::…boundary_max_valid, test_media::…[zip]) verified unrelated to this change
• New unit tests:
  • test_jira_client_sets_default_user_agent / test_confluence_client_sets_default_user_agent — default UA is set on the session
  • test_jira_client_custom_user_agent_overrides_default / equivalent for Confluence — custom_headers still wins
• Updated test_no_custom_headers_applied (Jira & Confluence) to expect the default UA being present

[github-actions]

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