feat(work_packages): support custom fields on create/update (port upstream #16)

[AndreaV-Lsi]

Problem / motivation

create_work_package / update_work_package cannot set custom fields.
OpenProject work packages routinely carry custom fields (customField1, customField2,
…) — cost centers, categories, list/enumeration values, etc. Today there is no way
to write them through the MCP server, which blocks real project-management use.

Source / attribution

Based on upstream PR AndyEverything#16 by @roromedia
("Fix custom fields not being sent in API payload"). That PR patches the legacy
monolith and adds only the client-side passthrough. This fork's tools build the
API data dict from explicit Pydantic model fields, so a client-only change would
be dead code — nothing would ever put customField* into data. So this issue ports
the client passthrough and adds the missing tool-level entry point. Author to be
credited in NOTICE.

Scope / proposed approach

• Client (src/client.py, create_work_package + update_work_package): after
  the standard fields, pass through any data key starting with customField —
  a {"href": …} value goes under _links (list/user/version-type fields), any
  other value is set directly (text/number/date). (This is feat(sprints): native sprint & backlog management tools (clean implementation) #16's logic, adapted to
  our form-based payload builder.)
• Tools/models (src/tools/work_packages.py): add an optional
  custom_fields: Dict[str, Any] field to CreateWorkPackageInput and
  UpdateWorkPackageInput, and merge it into data before calling the client.

Usage: {"custom_fields": {"customField1": "ACME", "customField5": {"href": "/api/v3/custom_options/12"}}}

Acceptance criteria

• Creating/updating a WP with custom_fields sets both a text/number custom field
  (direct value) and a list-type custom field (_links href) in the same request.
• Omitting custom_fields changes nothing (no customField*/empty _links noise).
• Network-free unit test: given custom_fields, the client payload places text
  values directly and href values under _links.
• Live smoke on ephemeral OpenProject: define a WP custom field, set it via the
  tool, read it back.

Out of scope

• No change to how custom fields are discovered (listing definitions) — write path
  only. Branches off main (independent of the flattened-tools stack / PR fix: make wrapped tool inputs robust to MCP clients that serialize args as JSON strings AndyEverything/openproject-mcp-server#22).