feat(client): make HTTP User-Agent header configurable

[jdforrester]

Summary:
The Phabricator/Phorge client previously hardcoded a single User-Agent string ("ModelContextProtocol/1.0 (Autonomous; ...)") across all three HTTP client construction sites. Some Phabricator/Phorge operators (notably Wikimedia) inspect the User-Agent header for rate-limiting and require identifying contact information for automated clients, which made the server unusable against those instances without patching.

This change introduces a new optional PHABRICATOR_USER_AGENT environment variable that, when set, overrides the default User-Agent sent on every outbound request. When unset, behavior is unchanged — the existing default string is preserved verbatim via a new DEFAULT_USER_AGENT constant in conduit/client/base.py, so the change is fully backwards compatible.

The new setting follows the exact same wiring pattern as the existing PHABRICATOR_PROXY and PHABRICATOR_DISABLE_CERT_VERIFY options: environment variable → PhabricatorConfig field → PhabricatorClient kwarg → httpx.Client header. Both stdio and HTTP/SSE transport modes forward the value.

Changes:

• Added DEFAULT_USER_AGENT constant in conduit/client/base.py as the single source of truth for the default header value
• Added user_agent: Optional[str] = None parameter to BasePhabricatorClient.init, EnhancedPhabricatorClient.init, and the backwards-compatible PhabricatorClient.init wrapper
• Added PHABRICATOR_USER_AGENT env var handling to PhabricatorConfig
• Forwarded config.user_agent through ConduitApp.get_client in both stdio and SSE code paths
• Added unit tests covering default and override behavior across all three client flavors (basic, enhanced-via-wrapper, direct enhanced)
• Documented the new env var in README.md and AGENTS.md alongside the existing optional PHABRICATOR_* variables

Key Features:

• Fully backwards compatible: unset env var preserves the original User-Agent string byte-for-byte
• Works in both stdio and HTTP/SSE transport modes
• Single source of truth for the default value — future changes to the default touch exactly one line
• Enables use against Phabricator/Phorge instances that require identifying User-Agent headers (e.g. Wikimedia) without source patches

Test Plan:

• Added test_user_agent_default: asserts DEFAULT_USER_AGENT is applied on the basic path, the enhanced path (triggered via timeout=60.0), and direct EnhancedPhabricatorClient construction
• Added test_user_agent_override: asserts a user_agent kwarg overrides the default on all three of the above paths
• Ran full test suite against phorge_debug Docker container per AGENTS.md instructions: 299 passed, 1 skipped (was 297 passed before the two new tests)
• Ran `pre-commit run -a`: ruff check, ruff format, and bandit all pass
• Manually verified via MCP client config that setting PHABRICATOR_USER_AGENT="MyOrg-Phabricator-MCP/1.0 (contact@example.org)" causes the expected header to be sent on outbound requests

[Copilot]

Pull request overview

Adds support for configuring the outbound HTTP User-Agent used by the Phabricator/Phorge client via a new PHABRICATOR_USER_AGENT environment variable, wiring it through config → client construction → httpx.Client headers.

Changes:

• Introduces DEFAULT_USER_AGENT and a user_agent override parameter in client constructors.
• Plumbs PHABRICATOR_USER_AGENT through PhabricatorConfig and ConduitApp.get_client (stdio + SSE paths).
• Adds unit tests and documentation updates for default vs overridden behavior.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
README.md	Documents PHABRICATOR_USER_AGENT usage.
AGENTS.md	Documents PHABRICATOR_USER_AGENT for dev/test workflows.
conduit/conduit.py	Reads env var into config and forwards to PhabricatorClient.
conduit/client/base.py	Adds DEFAULT_USER_AGENT and user_agent parameter to base client creation.
conduit/client/unified.py	Adds user_agent parameter and applies it to basic/enhanced httpx.Client headers.
conduit/client/tests/test_enhanced_client.py	Adds tests asserting default and override User-Agent behavior across client variants.

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[Copilot]

The new user_agent parameter is only applied when BasePhabricatorClient constructs its own httpx.Client (http_client is None). If a caller provides http_client, user_agent is silently ignored, which can be surprising given the parameter name/docstring; consider either documenting this explicitly, raising when both are provided, or applying the override to the provided client in a controlled way.