refactor: use shared config types from rattler_config

[wolfv]

Description

Continuation of #6144 ("refactor: switch to rattler_config") on a branch in this repo so it can be finished and merged — the original branch lives in a fork that couldn't be pushed to. The diff of #6144 applied cleanly onto current main; no functional deviations.

pixi_config starts consuming the shared leaf types from the rattler_config crate (0.5, crates.io) instead of maintaining its own copies:

• RepodataConfig / RepodataChannelConfig (including the tolerant deserializer that keeps deprecated keys like disable-jlap as warnings instead of hard errors)
• S3Options + the new S3OptionsMap newtype (IndexMap<String, S3Options>)
• ConcurrencyConfig (same defaults: solves = #CPUs, downloads = 50)
• ProxyConfig, BuildConfig / PackageFormatAndCompression, RunPostLinkScripts

All types are re-exported from pixi_config so external paths (pixi_config::S3Options, …) keep compiling. Merging goes through the rattler_config::config::Config trait; the old From<RepodataChannelConfig> for SourceConfig became a free function due to the orphan rule.

Follow-ups (flagged with TODO(rattler-config): promote comments, as discussed in #6144): promoting tls_root_certs, the allow_*_links trio and pinning_strategy are user-facing schema decisions and stay out of this PR. The upstream rattler_config redesign in conda/rattler#2557 (unknown-key warnings that work with extensions, generic set, shared config-file discovery) is the basis for the next step, where pixi's Config can become ConfigBase<PixiConfig>.

How Has This Been Tested?

• cargo check --workspace --all-targets passes.
• cargo test -p pixi_config: 53 passed, 0 failed (with proxy env vars scrubbed — see note below).
• cargo fmt --check and cargo clippy -p pixi_config -p pixi_api --all-targets clean.
• Note: test_config_merge_multiple is env-sensitive behind a proxy because rattler_config 0.5.2's ProxyConfig::default() reads proxy env vars; refactor(rattler_config)!: make the extension mechanism reliable and self-serving conda/rattler#2557 fixes this upstream (empty Default, explicit ProxyConfig::from_env()).

AI Disclosure

• This PR contains AI-generated content.
  • I have tested any AI-generated content in my PR.
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Tools: Claude Code

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.
• I have verified that changes that would impact the JSON schema have been made in schema/model.py (no schema impact — internal type moves only, serialized config format unchanged)

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