[vector_graphics_compiler] Fix rgb/rgba color parsing to support modern CSS syntax

[AlexDochioiu]

• Consolidate rgb() and rgba() parsing into single implementation
• Add support for space-separated color values
• Add support for percentage-based RGB values
• Add support for slash (/) separator before alpha channel
• Handle both comma and space delimiters
• Add comprehensive test coverage for various color formats

This PR solves https://github.com/flutter/flutter/issues/179261

Pre-Review Checklist

• [x] I read the Contributor Guide and followed the process outlined there for submitting PRs.
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• [x] I linked to at least one issue that this PR fixes in the description above.
• [x] I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or I have commented below to indicate which version change exemption this PR falls under[^1].
• [x] I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style, or I have commented below to indicate which CHANGELOG exemption this PR falls under[^1].
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[stuartmorgan-g]

• [ ] I linked to at least one issue that this PR fixes in the description above.
• [ ] I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or I have commented below to indicate which version change exemption this PR falls under1.

Thanks for the contribution! Once you've completed the checklist, please mark the PR as ready for review.

• [x] I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style, or I have commented below to indicate which CHANGELOG exemption this PR falls under1.

Please make sure to follow the linked style guide.

[AlexDochioiu]

@stuartmorgan-g Completed the checklist. This is my first PR to any flutter/dart repo. If anything is not exactly correct, please let me know and I'll address.

[AlexDochioiu]

Ping @stuartmorgan-g . Any chance we can get this fix reviewed/merged?

[stuartmorgan-g]

I can do an initial review, but this will need a review from the engine team. Review capacity tends to be low in December, which is probably why this hasn't been picked up yet.

[AlexDochioiu]

Thank you for the review @stuartmorgan-g . I completely re-wrote to a fresh implementation that checks strictly for the format and handles correctly all the cases (including weird ones such as R G,B,A -- where R G are space delimited) that seem to work on web.

I also added tests for a lot of invalid cases.

Bonus: I also added handling for

1. R,G,B values being double instead of int (which seems to be supported on web).
2. Values being out of bounds, which just clamp to the bounds on web rather than failing to parse

[AlexDochioiu]

/gemini review