feat(config): support custom provider headers

[aqilaziz]

Summary

• preserve headers in custom provider config and v11->v12 provider migration
• apply configured custom provider headers to main agent, runtime switches, and auxiliary clients
• document custom provider headers and add regression coverage

Fixes #9721

Tests

• python -m pytest -o addopts='' tests/agent/test_auxiliary_named_custom_providers.py -q
• python -m pytest -o addopts='' tests/hermes_cli/test_runtime_provider_resolution.py -q
• python -m pytest -o addopts='' tests/hermes_cli/test_config.py::TestCustomProviderCompatibility tests/hermes_cli/test_config_env_refs.py -q
• python -m pytest -o addopts='' tests/run_agent/test_provider_attribution_headers.py -q
• python -m pytest -o addopts='' tests/e2e/test_discord_adapter.py -q
• python -m pytest -o addopts='' tests/run_agent/test_provider_parity.py::TestDeveloperRoleSwap::test_developer_role_via_nous_portal tests/run_agent/test_provider_parity.py::TestBuildApiKwargsNousPortal::test_includes_nous_product_tags tests/run_agent/test_provider_parity.py::TestBuildApiKwargsNousPortal::test_uses_chat_completions_format -q
• python -m py_compile hermes_cli/config.py hermes_cli/runtime_provider.py agent/auxiliary_client.py run_agent.py tests/hermes_cli/test_config.py tests/hermes_cli/test_runtime_provider_resolution.py tests/run_agent/test_provider_attribution_headers.py tests/agent/test_auxiliary_named_custom_providers.py tests/e2e/conftest.py tests/run_agent/test_provider_parity.py
• python -m ruff check hermes_cli/config.py hermes_cli/runtime_provider.py agent/auxiliary_client.py run_agent.py tests/hermes_cli/test_config.py tests/hermes_cli/test_runtime_provider_resolution.py tests/run_agent/test_provider_attribution_headers.py tests/agent/test_auxiliary_named_custom_providers.py tests/e2e/conftest.py tests/run_agent/test_provider_parity.py
• git diff --check

[kshitijk4poor]

Closing as superseded by #41096, which adds model.default_headers support for custom OpenAI-compatible providers across both the main and auxiliary client paths (overriding the OpenAI SDK's User-Agent/X-Stainless-* headers that some gateways/WAFs reject — #40033). Thanks for the contribution! 🙏