Sync index format when uv add --index updates an existing index URL

[BitWeaverDev]

Summary

uv add --index <name>=<url> matches an existing index by name and overwrites its url, but it previously left the format key in place. An index declared as format = "flat" keeps that format even after being repointed at a Simple-API URL, silently producing an inconsistent configuration.

The add itself succeeds (the CLI-supplied index carries the correct Simple format), so the breakage only surfaces later — a subsequent uv lock reads the stale format = "flat" and fails to resolve.

Reproduction from #19759:

$ uv init --bare --name test
$ cat >> pyproject.toml <<EOF
[[tool.uv.index]]
name = "index"
url = "https://example.com/flat"
format = "flat"
EOF
$ uv add iniconfig --index index=https://pypi.org/simple
$ grep -A3 'tool.uv.index' pyproject.toml
[[tool.uv.index]]
name = "index"
url = "https://pypi.org/simple"
format = "flat"   # <-- stale, breaks `uv lock`

Changes

In PyProjectTomlMut::add_index, capture whether the URL is actually changing. When it is, sync the format to match the incoming index:

• write format = "flat" for flat indexes, and
• drop the format key entirely for Simple indexes (the default).

When the URL is unchanged, the existing format is left untouched, so entries that aren't being repointed keep their configuration.

Test

Two unit tests in pyproject_mut.rs, matching the module's existing test style:

• add_index_clears_format_on_url_update — repointing a flat index at a Simple URL removes the stale format key.
• add_index_preserves_format_when_url_unchanged — re-adding the same URL leaves format = "flat" intact.

Closes #19759

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