[maxches99]

Adds an orderedImports.onlyGroupTestableImports configuration option that restores the import grouping behavior of swift-format 602.0.0 and earlier, as suggested by @allevato in #1242.

Fixes #1242.

Motivation

#1013 had an unintended side effect on existing code: any import with a @testable attribute — even when it isn't the first attribute, e.g. @_spi(Testing) @testable import Foo — is now moved into the @testable group, and @_implementationOnly imports are moved into a new group of their own. This changed formatting output between 602 and 603 with no configuration escape hatch, which breaks projects that enforce formatting in CI.

Changes

• New option orderedImports.onlyGroupTestableImports (default false, so the current 603 behavior is unchanged). When true, the pre-603 classification is used: only an import whose first attribute is @testable is placed in the @testable group, no separate group is created for @_implementationOnly imports, and imports with any other attributes are grouped and sorted together with regular imports. The legacy classification is taken verbatim from the pre-OrderedImports should create separate group for @_implementationOnly … #1013 implementation.
• Robust decoding of OrderedImportsConfiguration. The struct relied on synthesized Codable, which throws on missing keys — so adding any new option would have broken every existing configuration file containing an orderedImports object (and a partial object such as {"includeConditionalImports": true} already failed to decode today). It now has an explicit init(from:) that defaults missing keys, consistent with how the top-level Configuration decodes.
• Documentation: rule doc comment (with RuleDocumentation.md regenerated via generate-swift-format) and Documentation/Configuration.md.
• Tests: the exact scenario from 602.0.0 and 603.0.0 produce different output with the same configuration #1242 (file is left untouched with the option enabled), sorting of attributed imports within the regular group, leading-@testable imports still being grouped, and decoding of partial orderedImports configuration objects.

Verification

On the reproduction from #1242, default behavior is unchanged, and with {"orderedImports": {"onlyGroupTestableImports": true}} the output matches 602.0.0 byte-for-byte. Full test suite passes.

[maxches99]

Gentle ping — rebased onto current main. This adds an option to restore the pre-603 import grouping in OrderedImports (#1242). @allevato, would you have a chance to review it? The workflow run will need approval.