fix(knowledge): show platform-accurate valid options in embedding type error

[sandikodev]

Issue

Closes #3139

Problem

On Linux ARM64, EmbeddingType::Best is not available — the candle module is excluded via #[cfg(not(all(target_os = "linux", target_arch = "aarch64")))]. When a user passes --index-type best on that platform, from_str correctly returns None, but the error message was misleading:

Invalid embedding type 'best'. Valid options are: fast, best

best is listed as a valid option but is not accepted on that platform, causing user confusion (as reported in the issue).

Fix

Use cfg to build the valid options string at compile time:

#[cfg(all(target_os = "linux", target_arch = "aarch64"))]
let valid = "fast";
#[cfg(not(all(target_os = "linux", target_arch = "aarch64")))]
let valid = "fast, best";
return Err(format!("Invalid embedding type '{}'. Valid options are: {}", s, valid));

On Linux ARM64 the error now reads:

Invalid embedding type 'best'. Valid options are: fast

Testing

# Run the targeted test
cargo test -p chat_cli invalid_embedding_type

# Run the full test suite to check for regressions
cargo test -p chat_cli

The test invalid_embedding_type_error_does_not_mention_best_on_linux_arm uses cfg to assert platform-specific behaviour:

• On Linux ARM64: EmbeddingType::from_str("best") returns None and the error message does not mention best
• On other platforms: EmbeddingType::from_str("best") returns Some (no change in behaviour)

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.