Migrate project management and CI workflows to uv

[theAnuragMishra]

What does this PR do, and why?

This PR migrates project dependency management and build configuration from pip/pipenv to uv. This improves developer experience by providing faster installs, reproducible environments via a lockfile, and various convenience commands provided by uv.

External discussion & connections

• Discussed in #zulip-terminal in #zulip-terminal > any plans to migrate to uv?
• Fully fixes #
• Partially fixes issue #
• Builds upon previous unmerged work in PR #
• Is a follow-up to work in PR #
• Requires merge of PR #
• Merge will enable work on #

How did you test this?

• Manually - Behavioral changes
• Manually - Visual changes
• Adapting existing automated tests
• Adding automated tests for new behavior (or missing tests)
• Existing automated tests should already cover this (only a refactor of tested code)

Self-review checklist for each commit

• It is a minimal coherent idea
• It has a commit summary following the documented style (title & body)
• It has a commit summary describing the motivation and reasoning for the change
• It individually passes linting and tests
• It contains test additions for any new behavior
• It flows clearly from a previous branch commit, and/or prepares for the next commit

[theAnuragMishra]

I'm well aware that README.md contains a lot of unrelated changes but most of them are caused by my editor formatting md according to modern idioms.
If needed, I can pick only relevant hunks.

[theAnuragMishra]

A bit surprising that one of the tests fails because pytest passes locally.

[Niloth-p]

Thanks for the PR!

Formatting changes and content changes do not go in the same commit. That is every reviewer's nightmare.

I understand that it's been a while since those files were updated, so this is natural. It might take a while for the formatting to get merged to main. Please try turning off the formatter's on-save trigger until then.

Or you could just save those files and make that the first commit, then your changes can go in the second commit. Making it possible to review your changes.

The failing test is a flaky test, pushing again should solve it.

[theAnuragMishra]

I didn't like the idea of making a format-only commit which only formats a few files. We can maybe do a full source formatting later.
For now I have removed all the formatting changes. The one large diff is uv.lock which is auto generated.
I think it's ready for review now!

[theAnuragMishra]

A few points I'd like to note:

• since we are migrating to uv (and not also supporting uv) , pipenv support has been completely removed
• users can however set up everything manually using pip if they don't want uv. They will just miss some of the nice things uv has.