refactor: replace pixi_git implementation with the unified rattler_git crate

[wolfv]

Description

Companion to conda/rattler#2590, which unifies the low-level git cache implementation between pixi, rattler and rattler-build.

pixi_git and rattler_git share a common ancestor (both derived from uv-git) but drifted apart: pixi_git grew git-LFS support, a lazy HTTP client, cloneable errors and a graceful GitHub fast-path fallback, while rattler_git grew submodule handling with relative-URL resolution and corrupted-cache recovery. All of pixi_git's extras are now upstreamed into rattler_git (see the rattler PR), and this PR makes pixi consume it.

pixi_git becomes a thin shim that re-exports rattler_git and keeps only the pixi-specific pieces:

• the PIXI_GIT_LFS environment variable (identical tri-state semantics, now parsed by rattler_git::source::lfs_enabled_from_env),
• default_checkout_options() — submodules on, LFS following PIXI_GIT_LFS,
• to_git_client() — bridges rattler_networking::LazyClient into rattler_git::LazyClient without losing laziness (the reqwest client is still only built if the GitHub fast path actually runs).

Because all consumers imported through pixi_git::…, no other pixi code changes except the single GitResolver::fetch call site, which now passes the checkout options explicitly.

pixi also gains the rattler_git features pixi_git lacked:

• relative submodule URLs (../sibling.git) are resolved against the real source URL instead of the local bare database path, so they no longer fail after git clone --local,
• protocol.file.allow=always for file-based submodules (Git ≥ 2.38.1),
• corrupted git caches are detected (git rev-parse --git-dir validation) and re-cloned instead of erroring.

The LFS integration tests moved to rattler_git together with the code they test.

⚠️ The [patch.crates-io] entries for rattler_git, rattler_build_source_cache and rattler_build_networking are temporary — they point at the companion branches (conda/rattler#2590, prefix-dev/rattler-build#2648) and must be replaced by released versions before merging.

How Has This Been Tested?

• cargo check --workspace --all-targets passes with the patched graph (this also compiles rattler-build's source cache against the new rattler_git).
• cargo test -p pixi_git -p pixi_compute_sources and the full pixi_command_dispatcher integration suite (32 tests, incl. git checkout/pinning tests against local fixtures) pass.
• The ported LFS tests pass in rattler_git itself (cargo test -p rattler_git on the rattler branch), covering PIXI_GIT_LFS parsing, pointer-vs-blob checkouts, and warm-cache LFS validation.
• cargo clippy -p pixi_git -p pixi_compute_sources --all-targets and cargo fmt are clean.

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 Code

We have source / git caches in rattler, rattler-build and Pixi. I think Pixi
"git cache" has drifted a bit from rattler's. Can you port everything needed
in Pixi to rattler, so that we can use one low level code base for this?
Then open a draft PR to Pixi and rattler with the changes, and possibly
rattler-build if needed. Things that might be different are related to lfs,
submodules, ..

Checklist:

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

🤖 Generated with Claude Code

https://claude.ai/code/session_013EJTWLNjUFMPPPoT8746Me

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