Improve String dictionary key strategy tests to verify nested value conversion

[ShivaHuang]

Update both tests to use a nested Codable struct as the dictionary value.

Motivation:

While reviewing #1526, we noticed encodingDictionaryStringKeyConversionUntouched and
decodingDictionaryStringKeyConversionUntouched only verify that String dictionary keys
are preserved, but don't check that key strategies are still correctly applied to nested
struct properties.

Modifications:

• Update decodingDictionaryStringKeyConversionUntouched to use DecodeMe3 values
• Update encodingDictionaryStringKeyConversionUntouched to use DecodeMe3 values
• Verify String dictionary keys bypass strategy while nested properties apply it
• Ensures consistent comprehensive test coverage for dictionary key behavior

Result:

Tests now verify that dictionary keys bypass the strategy AND nested properties apply it

Testing:

• encodingDictionaryStringKeyConversionUntouched
• decodingDictionaryStringKeyConversionUntouched

[jmschonfeld]

@swift-ci please test

[ShivaHuang]

Hi @kperryua, I believe there was some temperate issue in CI system, can you trigger testing again?

[kperryua]

@swift-ci please test

[ShivaHuang]

It looks like this failure is a CI infrastructure issue rather than a code problem - the Windows runner failed to connect to the Docker daemon (open //./pipe/docker_engine: The system cannot find the file specified), which is unrelated to the changes in this PR. All other recent PRs passed the same Windows (nightly-main) job without issue.

Could someone look into the CI runner, or alternatively, would merging main into this PR help trigger a fresh runner assignment?

[ShivaHuang]

The CI failures are unrelated to this PR's changes:

• Wasm SDK Build: Infrastructure failure — swift sdk install crashed (exit code 139) after 5 attempts. Not reproducible locally and not caused by any code change.
• Linux/Windows localizedName_103036605: Pre-existing DST flakiness in TimeZoneTests (already acknowledged in the codebase with a comment and addressed in Disable a timezone test #1862). Rebased onto main to pick up that fix.

[finagolfin]

@swift-ci test

[ShivaHuang]

Hi @kperryua, CI is now passing. Friendly ping in case this slipped through. Happy to make any changes if needed!