Improve test coverage

[InteractionEngineer]

I plan to improve the app and need to do some significant amount of changes to the infrastructure (especially likely to combine) eventually. Real people depend on this single client on the iOS platform and one of my highest goals is to only increase the value PayForMe brings to them and therefore prevent introducing new bugs. That's why one of my first bigger contribution to this codebase are many unit tests that ensure the current (correct) behavior of the app.

Changes were co-authored by Claude Sonnet 4.6 High and every line of code was manually reviewed, and altered if needed, by a real human (me).

[Copilot]

Pull request overview

This PR adds a substantial unit-testing safety net for PayForMe before larger refactors. It introduces shared test helpers, expands existing URL/AddProject tests with clearer naming and richer assertions, and adds new test suites covering URL request building, JSON decoding, bill parameter generation, sort modes, and balance calculation. A stray copyright line is also removed from ShareProjectQRCodeViewModel.swift.

Changes:

• New test files: TestHelpers (mock URL protocol + fixtures), NetworkRequestTests, JSONDecodingTests, BillTests, BillSortingTests, BalanceCalculationTests.
• Existing UrlExtensionsTests and AddProjectManuallyTests are simplified, renamed for intent, and given explanatory comments.
• Xcode project updated to include the new test files in the test target.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
PayForMeTests/TestHelpers.swift	Adds MockURLProtocol and Project/Bill/Person fixtures used across new tests.
PayForMeTests/NetworkRequestTests.swift	Verifies URL paths, auth headers, and HTTP methods for both backends via MockURLProtocol.
PayForMeTests/JSONDecodingTests.swift	Decodes representative server payloads into Bill, Person, and APIProject.
PayForMeTests/BillTests.swift	Asserts backend-specific shape of Bill.paramsFor and Bill.newBill() defaults.
PayForMeTests/BillSortingTests.swift	Covers BillListViewModel.SortedBy.sort for both modes and edge cases.
PayForMeTests/BalanceCalculationTests.swift	Validates BalanceViewModel.setBalances() math, including the zero-sum invariant.
PayForMeTests/UrlExtensionsTests.swift	Tightens existing tests and adds QR-dispatch and malformed-URL coverage.
PayForMeTests/AddProjectManuallyTests.swift	Renames tests for clarity and documents debounce timing.
PayForMe/Views/Projects/ShareProjectQRCodeViewModel.swift	Removes a single copyright line from the file header.
PayForMe.xcodeproj/project.pbxproj	Wires the new test files into the test target.

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