test: add comprehensive C# and Python unit tests for the PyMCU SDK

[Copilot]

The SDK had no tests of its own despite containing non-trivial logic across the IR layer, backend analysis passes, serialization, license validation, and Python plugin abstractions. This brings a self-contained test suite ported and adapted from the upstream PyMCU monorepo, extended with edge-case coverage specific to this package.

C# — tests/csharp/ (138 xUnit tests)

• IR/DataTypeTests.cs — SizeOf, IsSigned, StringToDataType (all type strings including const[T], ptr[T], PIORegister, null/unknown), GetPromotedType (same type, larger wins, mixed-signedness promotion at all widths)
• IR/TackyTests.cs — Record equality/defaults for every Val and Instruction subtype; ProgramIR/Function defaults and mutation
• Backend/Analysis/StackAllocatorTests.cs — Global allocation, local frame sequencing, param registration via call graph entry points, globals not re-allocated as locals, ISR function allocation, VariableSizes for all data widths, bit instructions, ArrayLoad/ArrayStore byte sizing
• Backend/Analysis/DynamicStackAllocatorTests.cs — Downward-growing offsets, params-before-body-locals ordering, 16-byte alignment invariant, unique offsets, temporaries, custom word sizes
• Backend/Serialization/IrSerializerTests.cs — Full JSON round-trip for every instruction and value type, including FlashData, MemoryAddress, FloatConstant, InlineAsm with operands
• Backend/License/LicenseResultTests.cs — All factory methods (Free, Ok, NotFound, Expired, WrongFamily)
• Backend/License/LicenseValidatorTests.cs — ResolveKey resolution order (explicit > env > whitespace), JWT parsing for valid/expired/wrong-family/all-families tokens, case-insensitive family matching

Python — tests/python/ (56 pytest tests)

• test_base_tool.py — _default_platform_key arch normalisation (amd64→x86_64, aarch64→arm64), _is_non_interactive (CI env, PYMCU_NO_INTERACTIVE, tty), verify_sha256, _extract_archive for tar.gz/bz2/zip including zip-slip path traversal protection for both tar and zip formats, base dir env override, version cache helpers
• test_backend_plugin.py — resolve_license_key full priority chain, BackendPlugin.supports (exact, prefix, case-insensitive, false-positive guard), validate_license default/override, LicenseStatus enum completeness
• test_toolchain_plugin.py — ToolchainPlugin.get_instance uses default_chip, get_ffi_toolchain default returns None, ExternalToolchain chip storage and assemble contract

Infrastructure

• tests/csharp/PyMCU.Backend.SDK.Tests.csproj — references the SDK project; pulls xUnit 2.7 + FluentAssertions 6.12
• PyMCU.SDK.slnx — test project added to the solution
• pyproject.toml — [tool.pytest.ini_options] block pointing at tests/python/