Enforce match_arm_blocks = true for consistent formatting

[pgherveou]

Summary

Flips match_arm_blocks from false to true to ensure all multi-line match arm bodies are wrapped in braces consistently.

Problem

With match_arm_blocks = false, rustfmt doesn't add braces to multi-line match arms, but it also doesn't remove existing braces. This means both styles are valid:

// Style A (no braces)
AccountIdOrAddress::AccountId(id) =>
    <T::AddressMapper as AddressMapper<T>>::to_address(id),

// Style B (with braces) - also valid, rustfmt won't change it
AccountIdOrAddress::AccountId(id) => {
    <T::AddressMapper as AddressMapper<T>>::to_address(id)
},

LLMs tend to produce Style B, which creates unnecessary diff noise in PRs.

Solution

Set match_arm_blocks = true to enforce Style B everywhere. Now there's exactly one valid style, eliminating the ambiguity.

Impact

556 files changed — this is a one-time formatting update. All future code will be consistently formatted.

Follow-up to #10939.

[pgherveou]

@bkchr follow up from #10939
these changes are also annoying with llm, should we go with this despite the big diff

[bkchr]

😿