fix(xml): handle rational types in ImageSpec roundtrip

[ayushactiveat]

Fixes #5571.

Description

Fixes two issues preventing rational types (like FramesPerSecond) from surviving an XML round-trip:

1. String parsing: TypeDesc::fromstring() didn't recognize rational2i or rational2f. Added these mappings to typedesc.cpp.
2. Value separator: The XML writer outputs 30000/1001 (with a slash), but the parser expected commas. Added logic in paramlist.cpp to handle the slash.

Tests

Added tests in typedesc_test.cpp for string parsing, and imagespec_test.cpp for the full XML round-trip. Both now verify that the type and the float value (29.97) survive to_xml() and from_xml().

Assisted-by: ByteAsk / GPT-5.4

Checklist:

• I have read the guidelines on contributions and code review procedures.
• I have read the Policy on AI Coding Assistants and added the Assisted-by line above.
• I have updated the documentation if my PR adds features or changes behavior. (N/A)
• I am sure that this PR's changes are tested in the testsuite.
• I have run and passed the testsuite in CI before submitting the PR. (Local build failed due to missing dependencies, relying on GitHub CI).
• My code follows the prevailing code style of this project.
• If I added or modified a public C++ API call, I have also amended the corresponding Python bindings. (N/A)

[linux-foundation-easycla]

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• ✅ login: ayushactiveat / name: Ayush Singh (3cf27a4)

[lgritz]

OK, I understand the fix about how rationals were written to XML as "N / D" when they should have been "N, D".

But what's rational2f about and why is it needed?

[ayushactiveat]

hi @lgritz , added rational2f because TypeDesc::c_str() can give that string for float based rationals, so just wanted the parser to handle everything the writer produces. if you like to keep focus only on the original issue for now, i can remove it, let me know