fix(config): support all configuration keys and subkeys in pixi config list

[aoright]

Description

This PR fixes an inconsistency where running pixi config list [KEY] failed for valid configuration keys and dotted subkeys (as noted in #6825).

Previously, partial_config() in crates/pixi_cli/src/config.rs hardcoded a fixed list of 12 keys, returning key must be one of: ... for valid configuration keys such as tls-root-certs, concurrency, detached-environments, pinning-strategy, run-post-link-scripts, s3-options, tool-platform, cache, experimental, and build. Furthermore, dotted subkeys across sections (e.g. cache.conda-packages, concurrency.solves, shell.change-ps1, proxy-config.http, repodata-config.disable-bzip2) were unsupported.

Key changes:

• Supported all top-level configuration keys in partial_config: tls-root-certs, concurrency, detached-environments, pinning-strategy, run-post-link-scripts, s3-options, tool-platform, cache, experimental, build.
• Supported nested dotted subkeys for shell.*, cache.*, proxy-config.*, pypi-config.*, repodata-config.*, index-config.*, experimental.*, concurrency.*, and s3-options.*.
• Added output subkey pruning in prune_subkeys to isolate requested subfields for non-Option sections like concurrency (solves vs downloads) and s3-options.<bucket> in both TOML and JSON output.
• Replaced the hardcoded static error list in partial_config with config.get_keys() so error diagnostics are dynamically synchronized with Config.
• Added "build" to Config::get_keys() and deserialization in Config::set().
• Exposed pub source_completion_scripts: Option<bool> on ShellConfig.
• Added comprehensive unit tests in crates/pixi_cli/src/config.rs and crates/pixi_config/src/lib.rs verifying all supported keys and subkeys.

Relates to #6825

How Has This Been Tested?

1. Unit tests in crates/pixi_cli/src/config.rs:
   • test_partial_config_tls_root_certs: verifies tls-root-certs is preserved.
   • test_partial_config_concurrency: verifies concurrency is preserved.
   • test_partial_config_concurrency_solves: verifies concurrency.solves is isolated in TOML/JSON.
   • test_partial_config_concurrency_downloads: verifies concurrency.downloads is isolated in TOML/JSON.
   • test_partial_config_cache_subkeys: verifies cache.conda-packages.
   • test_partial_config_shell_subkeys: verifies shell.change-ps1.
   • test_partial_config_proxy_subkeys: verifies proxy-config.http.
   • test_partial_config_pypi_subkeys: verifies pypi-config.index-url.
   • test_partial_config_repodata_subkeys: verifies repodata-config.disable-bzip2.
   • test_partial_config_index_subkeys: verifies index-config.base-url.
   • test_partial_config_experimental_subkeys: verifies experimental.conda-script.
   • test_partial_config_s3_options: verifies s3-options, s3-options.<bucket>, and subkey pruning.
   • test_partial_config_invalid_key_error: verifies dynamic diagnostic message with all valid keys.
   • test_partial_config_all_supported_keys: exhaustively checks that every key in config.get_keys() is accepted by partial_config().
2. Unit tests in crates/pixi_config/src/lib.rs:
   • test_alter_config: verifies build key deserialization in Config::set().
3. Executed test suite:
   • cargo test -p pixi_config (64 passed, 0 failed)
   • cargo test -p pixi_cli (203 passed, 0 failed)
   • cargo clippy -p pixi_config -p pixi_cli --all-targets (passed, zero warnings)
   • cargo fmt --check (passed)

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

Support all configuration keys and subkeys in `pixi config list [KEY]` and synchronize with `Config::get_keys()`.

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.