Add mjs_makeFlex to the MuJoCo C and Python APIs.

- Add mjs_makeFlex C API that wraps mjCFlexcomp::Make(), creating
  a flex with auto-generated bodies, joints, and equality constraints
  from parameters (type, dim, dof, count, cellcount, spacing, scale,
  radius, mass, inertiabox, equality, rigid, flatskin, elastic2d,
  pos, quat, origin, file, vfs).
- Add Python binding body.make_flex() in specs.cc with keyword args.
- Refactor flex_spec_provision.py to use make_flex() instead of XML
  string templating + separate spec + attach. Eliminates textwrap
  dependency.
- Merge user_flexcomp into user_model BUILD target to resolve
  circular dependency.

PiperOrigin-RevId: 922725304
Change-Id: Ice13787c45f3173407fec659f9590a6453bc65dd

doc/changelog.rst

@@ -5,11 +5,16 @@ Changelog
 Upcoming version (not yet released)
 -----------------------------------
 
+General
+^^^^^^^
+- Added :ref:`mjs_makeFlex`, a new C API function equivalent to the :ref:`flexcomp<body-flexcomp>` element for
+  programmatically creating flex objects with auto-generated bodies, joints, and equality constraints. Exposed as
+  ``body.make_flex()`` in Python.
+
 Bug fixes
 ^^^^^^^^^
 - Fixed a bug in the ``mjz`` :ref:`decoder <mjpDecoder>` where unnormalized paths would fail to be read.
 
-
 Version 3.9.0 (May 27, 2026)
 ----------------------------