Make client_id optional in vMCP OIDC config

[jhrozek]

client_id is only required for specific flows like token introspection with client credentials. For standard JWT validation using JWKS, only issuer and audience are needed.

This follows the same pattern as the previous change that made client_secret_env optional (d543c84).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 56.56%. Comparing base (20ecb03) to head (6ed551c).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2822      +/-   ##
==========================================
+ Coverage   56.55%   56.56%   +0.01%     
==========================================
  Files         320      320              
  Lines       30874    30872       -2     
==========================================
+ Hits        17460    17462       +2     
+ Misses      11911    11908       -3     
+ Partials     1503     1502       -1     

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