Chore: Add API breakage gate, platform CI matrix and changelog (Phase 0)

[Krivoblotsky]

What

Phase 0 of the maintenance plan: a safety net that makes "no breaking changes" and "builds on every declared platform" checkable in CI, plus the paperwork that goes with it.

• API Breakage workflow (.github/workflows/api-breakage.yml): runs swift package diagnose-api-breaking-changes against the latest semver tag on every pull request and push to main, writes the report to the job summary, and fails on every break not listed in .github/api-breakage-allowlist.txt. Manual runs can pass a different baseline.
• Swift Build workflow now covers what Package.swift declares: Linux in swift:5.10, swift:6.0 and swift:6.3 containers (build + test), macOS and iOS Simulator (test), tvOS, watchOS and visionOS (build) on macos-latest.
• CHANGELOG.md in Keep a Changelog format, seeded from the 0.4.9, 0.5.0 and 0.5.1 release notes, with the 0.5.1 API changes spelled out.
• CONTRIBUTING.md: an API stability section describing the additive-only policy, how the gate works, how to accept an unavoidable break, and how regenerated Components.Schemas types are treated.
• Test target compiles for iOS 13 again: testModerationsIterable used Regex, which needs iOS 16.
• One build warning fixed in ModelResponseEventsStreamInterpreter (optional interpolated into a debug log line).
• Linux with Swift 5.10 builds again: swift-corelibs-foundation only gained the async URLSession APIs in Swift 6, so URLSession did not satisfy our URLSessionProtocol there and the package did not compile on the toolchain it declares as its minimum. A Linux-only, pre-Swift-6 extension bridges dataTask(with:completionHandler:) to async/await, with cancellation.

Why

swift package diagnose-api-breaking-changes 0.5.0 reports 77 public API breaks shipped in 0.5.1: 71 inside the generated Components.Schemas (for example transcription timestamps changed from Float to Double, ImageGenTool.SizePayload lost its cases, responseId was removed from the response audio events) and 6 in hand-written types (two ChatCompletionMessageParam initializers and ChatResult.Choice.Message.Annotation removed). Nothing in CI could have caught them, and CI only ever ran one unpinned Linux toolchain even though the package declares Swift 5.10 and five Apple platforms.

Affected Areas

CI configuration and documentation, one debug-only log line, one test using replacingOccurrences(of:with:options:) instead of Regex, and one new internal Linux-only file (URLSession+AsyncCompatibility.swift) that compiles only with compiler(<6.0). No public API changes; the new gate confirms this against 0.5.1.

More Info

• The allowlist is intentionally empty. Every accepted break needs an entry there and a CHANGELOG line, and the file is emptied when the release containing the break is tagged.
• Components.Schemas stays public. The policy in CONTRIBUTING.md asks for shims on regeneration instead of silently accepting breaks; moving generated types out of the public surface is a 1.0 topic.
• swift-openapi-runtime 1.10.0 requires Swift 6.0 and 1.11.0+ requires Swift 6.1. Our Package.resolved pins 1.12.0, which a Swift 5.10 or 6.0 toolchain refuses outright, so the Linux 5.10 and 6.0 lanes delete the pin and resolve the way a consumer of this library does (they get 1.9.0 and 1.10.0). Consumers never use our Package.resolved, so this is the realistic check.
• Affected areas now include one Linux-only source file. Its code was type-checked on macOS with the platform condition removed, and the Linux 6.0 and 6.3 lanes, which use Foundation's own implementation, are unaffected.

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