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

[codecov]

Codecov Report

❌ Patch coverage is 0% with 172 lines in your changes missing coverage. Please review.
✅ Project coverage is 0.00%. Comparing base (fee6bb0) to head (30b524f).

Files with missing lines	Patch %	Lines
src/roboticstoolbox/tools/urdf/urdf.py	0.00%	92 Missing ⚠️
src/roboticstoolbox/models/URDF/URDFRobot.py	0.00%	80 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff           @@
##            main    #701    +/-   ##
======================================
  Coverage   0.00%   0.00%            
======================================
  Files        143     143            
  Lines      14115   14269   +154     
======================================
- Misses     14115   14269   +154     

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[petercorke]

Thank you for your interest in RTB, and for the thorough work here -- this is exactly the error reporting #673 was after. Since your environment couldn't build the C++ extension, I built and ran the full test suite against this myself (762 tests) to cover that gap -- zero regressions across every model the repo ships. Merging.