feat(site): Validate admin password

[cogk]

Shell characters can cause issues because they are not escaped in the bench new-site command

https://stackoverflow.com/questions/15783701/which-characters-need-to-be-escaped-when-using-bash/44581064#44581064

Example with abc$defgh&QSDF:

{
    "command": "docker exec -w /home/frappe/frappe-bench  bench-****-******-** bench new-site --no-mariadb-socket --mariadb-root-username _****************_limited --mariadb-root-password **************** --admin-password abc$defgh&QSDF --db-name _**************** ***************.example.cloud",
    "directory": "/home/frappe/benches/bench-****-******-**",
    "duration": "0:00:00.260719",
    "end": "2025-08-04 11:50:09.592477",
    "output": "/bin/sh: 1: QSDF: not found\nUsage: bench  new-site [OPTIONS] SITE\nTry 'bench  new-site --help' for help.\n\nError: Missing argument 'SITE'.",
    "returncode": 127,
    "start": "2025-08-04 11:50:09.331758",
    "status": "Failure",
    "traceback": "..."
}

[codecov]

Codecov Report

❌ Patch coverage is 77.77778% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 35.40%. Comparing base (6f82b05) to head (b7f5641).
⚠️ Report is 701 commits behind head on develop.

Files with missing lines	Patch %	Lines
press/press/doctype/site/site.py	77.77%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #2886   +/-   ##
========================================
  Coverage    35.39%   35.40%           
========================================
  Files          431      431           
  Lines        35773    35784   +11     
========================================
+ Hits         12662    12669    +7     
- Misses       23111    23115    +4     

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