mjcf parser: Fix unknown size vector parsing

[JafarAbdi]

I ran into std::__ios_failure: basic_ios::clear: iostream error errors while trying to parse g1.xml, after debugging it I found that it happens when parsing the keyframe's qpos attribute here.

This PR fixes it by changing the stream exception handling from failbit to badbit and simplifies vector parsing logic to better handle whitespace and newlines in MJCF files.

[hrp2-14]

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If this wasn't intentionnal, you can change the base branch of this PR to devel
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[github-actions]

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[JafarAbdi]

Should I revert this change? It's just meant as an extra test, otherwise a similar case is added in the test_get_unknown_size_vector_from_stream test

[jcarpent]

@MegMll @jorisv Could you handle this PR?