fix(urdf): report stage, element and line when URDF/xacro loading fails

[karkikamal098]

Summary

Part of #673 (URDF/xacro error reporting with line numbers).

A broken URDF or xacro used to surface as a bare KeyError('forearm'), an XML ParseError pointing into the xacro-expanded text the user never saw, or a ValueError with no element name. Loading now raises URDFError, a ValueError subclass so existing handlers keep working, whose message reports:

• the stage that failed: xml (not well-formed), xacro (expansion failed) or urdf (valid XML, invalid robot);
• the element chain, innermost first, e.g. <inertial> inside <link name="arm">;
• the line (and column for XML errors), plus xacro's include/macro stack for xacro failures;
• the path of a saved copy of the expanded URDF that the line number refers to.

Undefined parent/child link references, a missing joint type, missing <mass>/<inertia> and duplicate names now say which joint or link is at fault. A missing file raises FileNotFoundError with the resolved path instead of failing inside xacrodoc. URDFError is exported from roboticstoolbox.tools.urdf.

The happy path is unchanged: the wrappers only convert exceptions that already escaped.

Related issue

Part of #673.

Checklist

• PR title follows Conventional Commits
• Tests pass locally: not run, my machine has no C++ toolchain yet, so I could not build the package. All changed files compile and lint clean under the repo's ruff rules; CI will run the suite.
• Added tests: tests/test_urdf_errors.py, 13 cases covering malformed XML in source and expanded text, undefined property, missing package, undefined link, missing/invalid attributes, duplicate names, missing file, and a valid-string regression
• Type hints use modern syntax; docstrings in reST
• PR is small and focused
• No project-specific test or data files added