fix(cli): fail fast when --proxy has no or an unsupported protocol

[lemon0333]

Fixes #453

--proxy localhost:1234 (a proxy address without a protocol) appeared to fail silently. proxy-agent only rejects a missing/unsupported protocol lazily, on the first request, so the real error (Unsupported protocol for proxy URL) was only visible with -vvv, and users otherwise saw a misleading downstream failure (e.g. "need credentials").

This validates the proxy address up front in ProxyAgentProvider.create() and throws a clear, actionable error, reusing proxy-agent's own exported supported-protocol map (so the accepted set can't drift):

• no protocol / unparseable (localhost:1234, 1.2.3.4:8080) → Invalid proxy address '<addr>': it must be a URL that includes a protocol, e.g. 'http://<addr>'
• unsupported protocol (ftp://…) → Unsupported protocol '<proto>' in proxy address '<addr>'. Supported protocols are: …

A valid address (http://…, https://…, socks5://…, pac+http://…) is unchanged. Added unit tests for validateProxyAddress and ProxyAgentProvider.create() (10 tests, all passing).

Checklist

• This change contains a major version upgrade for a dependency and I confirm all breaking changes are addressed
  • Release notes for the new version:

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.34%. Comparing base (88f6ad1) to head (0f721a1).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1843      +/-   ##
==========================================
+ Coverage   90.31%   90.34%   +0.02%     
==========================================
  Files          80       80              
  Lines       12159    12195      +36     
  Branches     1725     1732       +7     
==========================================
+ Hits        10982    11018      +36     
  Misses       1143     1143              
  Partials       34       34              

Flag	Coverage Δ
suite.unit	90.34% <100.00%> (+0.02%)	⬆️

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