fix(module): prevent crash when resolving bun:main before entry_po… (#27027)

…int.generate()

`ServerEntryPoint.source` defaults to `undefined`, and accessing its
`.contents` or `.path.text` fields before `generate()` has been called
causes a segfault. This happens when `bun:main` is resolved in contexts
where `entry_point.generate()` is skipped (HTML entry points) or never
called (test runner).

Add a `generated` flag to `ServerEntryPoint` and guard both access
sites:
- `getHardcodedModule()` in ModuleLoader.zig (returns null instead of
crashing)
- `_resolve()` in VirtualMachine.zig (falls through to normal
resolution)

### What does this PR do?

### How did you verify your code works?

Co-authored-by: Claude Bot <claude-bot@bun.sh>
Co-authored-by: Claude <noreply@anthropic.com>

src/bun.js/ModuleLoader.zig

@@ -1140,14 +1140,14 @@ export fn Bun__runVirtualModule(globalObject: *JSGlobalObject, specifier_ptr: *c
 fn getHardcodedModule(jsc_vm: *VirtualMachine, specifier: bun.String, hardcoded: HardcodedModule) ?ResolvedSource {
     analytics.Features.builtin_modules.insert(hardcoded);
     return switch (hardcoded) {
-        .@"bun:main" => .{
+        .@"bun:main" => if (jsc_vm.entry_point.generated) .{
             .allocator = null,
             .source_code = bun.String.cloneUTF8(jsc_vm.entry_point.source.contents),
             .specifier = specifier,
             .source_url = specifier,
             .tag = .esm,
             .source_code_needs_deref = true,
-        },
+        } else null,
         .@"bun:internal-for-testing" => {
             if (!Environment.isDebug) {
                 if (!is_allowed_to_use_internal_testing_apis)