Add provider configuration to Demo

[GtechGovind]

What

• Add OpenAI, Gemini, and Custom provider choices to the Demo configuration UI.
• Persist the selected provider, base URL, and API key across launches.
• Parse custom base URLs into scheme, host, port, and base path, with validation for unsupported URLs and invalid ports.
• Reconfigure every Demo client through one middleware-preserving factory and use relaxed decoding for compatible providers.
• Add focused tests for provider presets, parsing behavior, configuration mapping, and URL validation.

Why

The Demo currently hardcodes OpenAI's default endpoint. This allows it to exercise Gemini's OpenAI-compatible endpoint or a custom endpoint without editing source code.

Closes #308.

Affected Areas

• Demo API configuration UI and persistence
• Demo client construction and reconfiguration
• DemoChat provider endpoint parsing and tests

Testing

• xcodebuild -scheme DemoChat -destination 'platform=iOS Simulator,name=iPhone 17,OS=26.5' test — 8 test functions / 15 test invocations passed.
• xcodebuild -project Demo/Demo.xcodeproj -scheme Demo -destination 'generic/platform=iOS Simulator' CODE_SIGNING_ALLOWED=NO build — passed.
• swift test — 187 XCTest tests and 25 Swift Testing tests passed.
• git diff --check — passed.

Direct swift test --package-path Demo/DemoChat on macOS remains blocked by existing ExyteChat package metadata: ExyteChat declares macOS 10.13 while ActivityIndicatorView and Kingfisher require macOS 10.15. The focused DemoChat tests pass through the supported iOS simulator destination above.