fix(http): preserve arch underscores in platform keys

[jdx]

Fixes incorrect conversion of platform key underscores to dashes in flat keys.\n\n- Preserves arch underscores (e.g., x86_64) while still normalizing os_arch to os-arch.\n- Adds unit test to prevent regression.\n\nThis resolves mismatches with expected platform keys and improves suggestion accuracy.

[Copilot]

Pull Request Overview

This PR fixes a bug in platform key normalization where all underscores were being converted to dashes, incorrectly transforming architecture names like "x86_64" to "x86-64". The fix preserves underscores within architecture names while still converting the OS/arch separator underscore to a dash.

• Updates the platform key normalization logic to only convert the first underscore (OS/arch separator) to a dash
• Adds comprehensive unit test to verify the fix works for various platform configurations
• Maintains backward compatibility with existing nested key patterns

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[Copilot]

The logic assumes only one OS/arch separator underscore, but this could incorrectly handle platform parts like 'linux_musl_x86_64' where 'musl' is part of the OS identifier. The current implementation would produce 'linux-musl_x86_64' instead of the potentially expected 'linux-musl-x86_64' or preserve the original format.

Suggested change

	let platform_key = if let Some((os_part, rest)) = platform_part.split_once('_') {
	format!("{os_part}-{rest}")
	let platform_key = if let Some(idx) = platform_part.rfind('_') {
	let (os_part, arch_part) = platform_part.split_at(idx);
	// os_part ends at idx, arch_part starts with '_'
	format!("{}-{}", os_part, &arch_part[1..])

[github-actions]

Hyperfine Performance

mise x -- echo

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
mise-2025.9.2 x -- echo	18.8 ± 0.4	18.1	21.1	1.00 ± 0.05
mise x -- echo	18.7 ± 0.9	18.1	34.5	1.00

mise env

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
mise-2025.9.2 env	18.2 ± 0.4	17.6	22.8	1.00
mise env	18.3 ± 0.3	17.8	19.6	1.01 ± 0.02

mise hook-env

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
mise-2025.9.2 hook-env	17.8 ± 0.4	17.1	20.2	1.01 ± 0.02
mise hook-env	17.5 ± 0.2	17.1	18.8	1.00

mise ls

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
mise-2025.9.2 ls	16.0 ± 0.2	15.6	16.8	1.00 ± 0.03
mise ls	16.0 ± 0.4	15.4	22.1	1.00

xtasks/test/perf

Command	mise-2025.9.2	mise	Variance
install (cached)	164ms	✅ 100ms	+64%
ls (cached)	61ms	61ms	+0%
bin-paths (cached)	65ms	65ms	+0%
task-ls (cached)	487ms	467ms	+4%

✅ Performance improvement: install cached is 64%