fix: (de)serialize info.repodata_revisions as dictionary

[baszalmstra]

Description

Updates info.repodata_revisions to match the latest CEP draft (conda/ceps#146), which describes it as a dictionary keyed by revision identifier (e.g. "v3") rather than a list. Both the regular and sharded repodata now read and write this dictionary form, and the in-memory representation is a revision-keyed map so a revision cannot appear twice.

The Python indexing bindings accept the same dictionary shape for repodata_revisions, with timestamps given as datetime values for consistency with the rest of the API.

@danyeaw

How Has This Been Tested?

• Unit and integration tests across the affected crates, including a regression test that reads the dictionary form from a repodata file in the test data and asserts it parses correctly.
• Python unit test covering the dictionary input and the resulting output.
• Formatting and linting pass for both Rust and Python.

AI Disclosure

• This PR contains AI-generated content.
  • I have tested any AI-generated content in my PR.
  • I take responsibility for any AI-generated content in my PR.

Tools: Claude

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added sufficient tests to cover my changes.