fix(accounting): filter users by name

[hnotshe]

Summary

Fix sacctmgr show user name=X returning every configured user.

The CLI now forwards name= and user= through the accounting RPC. ListUsersRequest gains an additive user field, and the accounting query applies exact user and account filters server-side.

Closes #418.

Testing

• Full Linux validation: formatting, SPDX headers, cargo-deny, clippy, all-target build, and complete test suite
• Focused sacctmgr parameter regression test
• PostgreSQL integration test covering user-only, account-plus-user, and no-match filtering
• Independent reviewer: PASS

[yansun1996]

Clean, correct, and backward-compatible. The new proto field user = 2 is a genuinely free tag (additive, no wire-compat break), the list_users query rewrite preserves the columns, DISTINCT ON, and ORDER BY, empty-string from existing callers maps to None so list-all is preserved, and the filters are parameter-bound. A couple of minor Slurm-parity notes inline.

[yansun1996]

Rebased onto the latest main — there were a few conflicts in the accounting tests where recent changes to add_user (association limits) and list_users overlapped with this PR's signature changes; resolved by keeping both sets of tests and threading the new filter argument through.

Also verified the fix end-to-end on a 2-node cluster with accounting enabled:

• sacctmgr show user name=<user> returns only that user
• sacctmgr show user user=<user> (alias form) returns only that user
• sacctmgr show user name=<user> account=<acct> applies both filters
• sacctmgr show user account=<acct> returns just that account's users
• a non-existent name returns an empty result (header only), no error

All green — build, clippy, fmt, and the unit tests pass on the rebased branch.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 15.32258% with 105 lines in your changes missing coverage. Please review.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #447      +/-   ##
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- Coverage   71.70%   71.62%   -0.08%     
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  Files         153      153              
  Lines       50708    50797      +89     
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+ Hits        36359    36380      +21     
- Misses      14349    14417      +68     

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