refactor: replace hand-rolled string→bool parsing with str_is_truthy/str_to_bool

[Copilot]

Consolidates scattered string-to-boolean parsing across the codebase into lance_core::utils::parse utilities, eliminating inconsistent ad-hoc implementations.

Changes

• New utility: str_to_bool(val: &str) -> Option<bool> — for cases where an unrecognized value should return None so a caller-side default takes effect. Truthy: 1/true/on/yes/y; falsy: 0/false/off/no/n; everything else: None.

• Replaced hand-rolled patterns in 16 files across lance-core, lance-encoding, lance-index, lance-io, lance-table, lance-namespace-impls, and lance crates:

  • v == "true" / v.to_lowercase() == "true" / v.eq_ignore_ascii_case("true") → str_is_truthy(v)
  • matches!(s.to_lowercase().as_str(), "true" | "1" | "yes") → str_is_truthy(s)
  • v.parse::<bool>().ok() (config properties) → str_to_bool(v)
  • !matches!(v, "" | "0" | "false" | "off" | "no") → str_is_truthy(v.trim())

• Added unit tests for str_to_bool covering truthy, falsy, and unrecognized inputs.

Intentionally unchanged

• lance-arrow/src/schema.rs — lance-core depends on lance-arrow; importing back would create a circular dependency.
• lance/src/dataset/fragment.rs — checks if a SQL predicate string is literally "true", a semantically distinct use case.
• lance/src/index/vector/details.rs — uses a generic parse::<T: FromStr> helper, not a string-bool-specific path.

• Fixes chore: replace string to boolean parsing with lance_core::utils::parse::str_is_truthy #8674

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[wjones127]

@copilot There is a failing test you need to addresss:

thread 'credentials::tests::test_has_credential_vendor_config' (31985) panicked at rust/lance-namespace-impls/src/credentials.rs:770:9:
assertion failed: !has_credential_vendor_config(&props)

[Copilot]

@copilot There is a failing test you need to addresss:

...

Fixed in the latest commit. has_credential_vendor_config was originally using .eq_ignore_ascii_case("true") (only accepts "true"/"TRUE", not "yes" or other truthy aliases). Replacing it with str_is_truthy inadvertently expanded the accepted values, breaking the test. Reverted it back to the original .eq_ignore_ascii_case("true") check, and moved the str_is_truthy import inside the #[cfg(feature = "credential-vendor-aws")] function where it's still needed.