[PM-33163] Fix admin panel version fetch error handling

[boris324]

Fixes #7142

Summary

Improved error handling in GetInstalledWebVersion to catch all failure scenarios that occur behind reverse proxies:

• TaskCanceledException: Request timeouts (common behind slow proxies)
• JsonException/KeyNotFoundException: Malformed or unexpected responses
• Non-success HTTP status: Now logs a warning instead of silently returning "-"

Previously only HttpRequestException was caught, so timeouts and parse errors would result in unhandled exceptions, leaving the version display stuck on the spinner.

Test plan

• Deploy behind a reverse proxy and verify the admin panel shows "Unable to fetch installed version" instead of hanging
• Verify normal version fetch still works when web vault is accessible
• Check logs for appropriate error/warning messages

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[bitwarden-bot]

Thank you for your contribution! We've added this to our internal Community PR board for review.
ID: PM-33163
Link: https://bitwarden.atlassian.net/browse/PM-33163

Details on our contribution process can be found here: https://contributing.bitwarden.com/contributing/pull-requests/community-pr-process.