fix(interpreter): expand command substitutions in assoc array keys

[chaliy]

Summary

• Associative array assignments where the key is a command substitution (e.g. m["$(echo hello)"]="world") silently produced an empty key
• Add async expand_assoc_key() that parses the subscript as a full Word and expands it with expand_word() when it contains $( or backtick
• Add spec test assoc_key_command_substitution

Test plan

• New spec test assoc_key_command_substitution passes
• All 1811 bash spec tests pass (100%)
• Full cargo test --all-features passes
• cargo fmt --check clean
• cargo clippy -- -D warnings clean

Closes #872

[Copilot]

Pull request overview

Fixes a bash-compatibility bug in Bashkit’s interpreter where associative array assignments using command substitution in the subscript (e.g. m["$(cmd)"]=...) would incorrectly resolve to an empty key instead of the expanded command output.

Changes:

• Add async associative-key expansion (expand_assoc_key) and use it for associative array assignments.
• Add a new bash spec case covering command substitution in associative array keys.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
crates/bashkit/src/interpreter/mod.rs	Expands associative array assignment keys via full word expansion when command substitution is present.
crates/bashkit/tests/spec_cases/bash/assoc-arrays.test.sh	Adds a spec test asserting correct key expansion for m["$(echo hello)"]="world".

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[Copilot]

In expand_assoc_key, the s.contains('')check looks redundant/dead for assignment subscripts because the lexer rewrites backtick substitutions to$(` during tokenization. Consider dropping the backtick condition (and adjusting the docstring accordingly) to avoid suggesting that the word parser here understands raw backticks.

Suggested change

	/// Falls back to `expand_variable_or_literal` for keys without `$(` or backtick.
	async fn expand_assoc_key(&mut self, s: &str) -> Result<String> {
	if s.contains("$(") || s.contains('`') {
	/// Falls back to `expand_variable_or_literal` for keys without `$(`.
	async fn expand_assoc_key(&mut self, s: &str) -> Result<String> {
	if s.contains("$(") {