Handle the request-public plugin stanza (#9)

[pscheid92]

The plugin client (HandleCommonStanza in PluginRecipient / PluginIdentity) handled msg, request-secret, and confirm, but not request-public — so a plugin asking the user for a non-secret value got unsupported and the operation failed. See #9.

Change

Handle request-public alongside request-secret, matching the reference plugin/client.go (which pairs them and distinguishes by type). The IPluginCallbacks.RequestValue(prompt, secret) plumbing already existed, so request-public simply calls it with secret: false:

case "request-secret":
case "request-public":
    var value = callbacks!.RequestValue(Encoding.UTF8.GetString(body), secret: type == "request-secret");
    conn.WriteStanza("ok", [], Encoding.UTF8.GetBytes(value));
    break;

It's also added to the no-callbacks guard, so it replies fail (per spec) when the client has no UI.

Tests

New coverage for both recipient and identity paths: the callback is invoked with secret: false, the value is returned via ok, and fail is sent when no callbacks are wired. Full suite green (Age.Tests 348/348, Age.TestKit 143/143, 0 skipped); runs on the 3-OS matrix.

Fixes #9

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.87%. Comparing base (61a9546) to head (a672d59).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #14      +/-   ##
==========================================
+ Coverage   91.57%   91.87%   +0.29%     
==========================================
  Files          38       38              
  Lines        2338     2338              
  Branches      310      310              
==========================================
+ Hits         2141     2148       +7     
+ Misses        139      133       -6     
+ Partials       58       57       -1     

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