Replace SafeLoader monkey-patch with dedicated YAML loader subclass

[rafal-gorecki]

Bug

rqt_tf_tree crashes with RecursionError when the TF tree launched before nodes publishing tf.

File ".../rqt_tf_tree/dotcode_tf.py", line 100, in default_tftree_construct_mapping
    data = self.construct_mapping_org(node, deep)
[Previous line repeated 984 more times]
RecursionError: maximum recursion depth exceeded

Reproducer

1. ros2 run rqt_tf_tree rqt_tf_tree
2. Click Refresh.
3. Launch some tf publisher
4. Click Refresh.

Error fires automatically on first display when rqt is started before any TF
publisher exists; rqt internally retries dotcode generation, which triggers
the second invocation.

Why the fix works

Previous code mutated yaml.SafeLoader.construct_mapping globally on every
call and unconditionally saved the "original" into construct_mapping_org.
After the second call, the saved "original" was itself the previous override,
so the override called itself — recursion.

The fix replaces the runtime mutation with a dedicated
_TfTreeYamlLoader(yaml.SafeLoader) subclass that registers the
int-key-stringifying mapping constructor once, at module import. No per-call
mutation → no idempotency concern → bug fixed by construction. Bonus: the
global yaml.SafeLoader is no longer polluted for other modules in the same
process.

[arne48]

Thank you very much for your fix @rafal-gorecki.
All contributions/improvements are always highly appreciated.
It seems that the PR #58 also addressed this issue and was recently merged.
However, your approach might help to improve the readability/structure of the code.
So if you would rebase your PR we could also merge your solution.

[rafal-gorecki]

Thanks @arne48!
Rebased onto latest humble. Since #58 already fixes the crash, this now sits on top of it as a structural improvement — replacing the global SafeLoader monkey-patch with a dedicated _TfTreeYamlLoader subclass, so the shared loader is no longer mutated.

[arne48]

Thank you very much for the fast update.
This is a now a nice structural upgrade.