Fully qualify stdlib types (Error, Encoder, Decoder) in generated code

[william-laverty]

Summary

Fixes generated code to emit Swift.Error, Swift.Encoder, and Swift.Decoder instead of unqualified Error, Encoder, and Decoder. This prevents compilation errors when user modules define their own types with those names.

Changes

Source changes

• translateCodable.swift: Changed .member("Error") → .member(["Swift", "Error"]), .member("Encoder") → .member(["Swift", "Encoder"]), .member("Decoder") → .member(["Swift", "Decoder"])
• translateBoxedTypes.swift: Same changes for boxed type Codable conformances

Test updates

• Updated Petstore/Types.swift reference source to expect fully qualified types
• Updated SnippetBasedReferenceTests.swift to match

Before / After

// Before
var errors: [any Error] = []
public init(from decoder: any Decoder) throws { ... }
public func encode(to encoder: any Encoder) throws { ... }

// After  
var errors: [any Swift.Error] = []
public init(from decoder: any Swift.Decoder) throws { ... }
public func encode(to encoder: any Swift.Encoder) throws { ... }

Closes #719

[czechboy0]

Thank you, @william-laverty 🙏

[FranzBusch]

We should probably think about generating code with full module qualifiers once 6.3 is released to avoid any ambiguity.

[czechboy0]

@william-laverty please rerun the formatter, the CI is failing on that

[czechboy0]

I think you need to revert all those formatting changes, and then run the Swift 6.2-aligned swift-format using the act command in the CONTRIBUTING guide. That will skip the doc files and other files we don't want to reformat.

In other words, no files where you didn't change code should be modified in this PR.

[william-laverty]

Reverted all formatting-only changes — the diff now only touches files where code actually changed (the Swift. qualification in Sources + corresponding test expectations). Sorry about the noise from the earlier swift-format runs.