feat(admin): safely delete OIDC providers

[cxymds]

Summary

• add rc admin idp openid delete <ALIAS> <PROVIDER_ID> with explicit interactive or --yes confirmation
• fetch and display a typed, secret-free provider summary before deletion
• fail closed for environment-managed providers and unsupported server capabilities before mutation
• use a bounded typed DELETE path with deterministic not-found, auth, conflict, unsupported, and server error classes
• emit output-v3 success data without leaking secret values or server error bodies

BREAKING contract update

The protected command reference is updated to document the new delete subcommand and its mandatory confirmation contract. Existing commands and output schemas remain backward compatible.

Validation

• cargo fmt --all --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace
• cargo test --test admin_oidc (20/20)
• git diff --check

Closes rustfs/backlog#1491
Refs rustfs/backlog#1379