Split tests/server/interactions.cpp (5,548 lines, 193KB) into 3 smaller
files to avoid MSVC C1060 "compiler is out of heap space" error on GitHub
Actions Windows CI.

Issue: Windows CI failed at interactions.cpp:2307 with C1060 during both
Debug and Release builds. The file was too large for MSVC to compile in
CI's constrained environment, despite compiling fine locally with the same
compiler version (MSVC 14.44.35207).

Solution: Split into 3 parts following the same pattern as commit 54dc027
which explicitly "split large test files for maintainability":

- interactions_part1.cpp (55 tests, 1,989 lines) - Core Tools/Resources/Prompts
- interactions_part2.cpp (59 tests, 2,104 lines) - Data types/Validation
- interactions_part3.cpp (50 tests, 1,741 lines) - Server capabilities/Notifications

All 164 tests accounted for and passing locally (3/3 tests, 0.04s).

Related: PR #11 (43e5e64) which introduced the large interactions.cpp file.
This change maintains all test coverage while fixing Windows CI builds.

Windows CI was hitting C1060 "compiler is out of heap space" even after
splitting test files to 1,200 lines. Root cause: GitHub's windows-latest
runners have only 16GB RAM, and parallel cl.exe processes exhaust memory
with template-heavy code.

Solution (based on Microsoft docs + community solutions):
1. Add `-A x64` to ensure 64-bit toolset (32-bit has ~2-3GB limit)
2. Use `/m:4` to limit MSBuild to 4 worker processes
3. Use `/p:CL_MPCount=1` to disable cl.exe's internal /MP flag

This reduces peak compiler memory usage while still maintaining reasonable
build performance (~4x parallelism at MSBuild level).

Refs:
- https://learn.microsoft.com/en-us/cpp/error-messages/compiler-errors-1/fatal-error-c1060
- appveyor/ci#742

The previous commit added -A x64 flag, but GitHub Actions cache
had a build directory configured without it. CMake errors when the
generator platform changes.

Fix: Delete CMakeCache.txt before configuring to allow clean
reconfiguration with the new x64 platform flag.

Previous attempt only removed CMakeCache.txt, but CMake also caches
configuration in the CMakeFiles directory. Both need to be removed
when changing generator platform flags.

The cached build/_deps contains FetchContent dependencies compiled
without -A x64 flag. Adding x64 to cache key forces fresh build of
dependencies with correct platform configuration.

Deeply nested JSON initializers in test files (9+ levels) cause MSVC
to exhaust compiler heap even with /m:4. Switching to /m:1 for maximum
memory conservation on GitHub's 16GB Windows runners.

The 9-level nested JSON initializer at line 549 exhausts MSVC compiler
heap even with single-threaded builds on GitHub's 16GB Windows runners.

Solution: Build the nested schema programmatically instead of using
nested initializers. This significantly reduces compiler memory pressure
while producing identical runtime results.

Tested locally: All 30 Part 2a tests pass.