[codex] Add BigModel China provider

[a19q3]

Summary

• Add a separate bigmodel OpenAI-compatible provider for Zhipu BigModel mainland China, defaulting to glm-5.1.
• Keep the existing zai provider on its current Z.AI endpoint/default so overseas users are not rerouted.
• Wire bigmodel through login metadata, CLI provider selection, auth source lookup, report/login picker surfaces, and static model fallbacks.
• Keep BigModel static models visible when the live /models catalog is incomplete.
• Preserve Kimi coding endpoint header behavior while ensuring BigModel is not treated as a Kimi/coding-agent endpoint.
• Lock configured default providers against cross-provider failover so missing credentials fail clearly instead of silently billing another provider.

Validation

• cargo fmt
• cargo test -p jcode-provider-metadata matrix_login_provider_aliases_resolve_to_canonical_ids
• cargo test -p jcode test_provider_choice_arg_values
• cargo test -p jcode test_provider_choice_aliases_parse
• cargo test -p jcode auth_issue_profile_metadata_matches_direct_provider_endpoints
• cargo test -p jcode test_kimi_coding_header_detection_matches_endpoint_and_model
• cargo test -p jcode bigmodel_static_models_remain_visible_when_live_catalog_is_incomplete
• cargo test -p jcode named_builtin_profile_without_env_file_uses_builtin_env_file
• cargo test -p jcode test_unconfigured_config_default_profile_stays_active_instead_of_using_other_profile_key

Local smoke checks confirmed jcode --no-update provider current resolves to Zhipu BigModel / glm-5.1 when configured, model list -p bigmodel shows glm-5.1, and a glm-5.1 BigModel request succeeds with saved credentials.