Centrally define the A2UI extension URI in typescript code

Currently, there are 6 places where we refer to the string `https://a2ui.org/ext/a2a-ui/v0.8` after https://github.com/google/A2UI/pull/209

We should have one place, so that it doesn't get out of sync, and in case we change our mind about what that string should be.

There should be some central place we refer to the extension name. There might also be an opportunity to share a bit more related logic around extension advertisement.

Search the codebase for the string 'https://a2ui.org/ext/a2a-ui/v0.8' to locate the six references. Create a constant in a central file, perhaps in a utilities or constants module, and replace all occurrences. Verify the changes by running existing tests to ensure no functionality is broken.

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