[22/44] Enable TS strict mode in block-editor

Description

Part of the strict-mode rollout epic #35932. Enable TypeScript strict mode for the block-editor project.

• Dependency layer: 6 (0 = leaf library, 10 = top-level app)
• Internal dependents: 5 (dotcms-block-editor, dotcms-ui, edit-content, edit-ema-ui, portlets-edit-ema-portlet)
• Rollout order: 22 / 44

Acceptance Criteria

• The six strict flags are added to libs/block-editor/tsconfig.json — the project's own config, not tsconfig.spec.json and not tsconfig.base.json, which stays "strict": false:
  forceConsistentCasingInFileNames, strict, noImplicitOverride, noPropertyAccessFromIndexSignature, noImplicitReturns, noFallthroughCasesInSwitch.
• tsc -p libs/block-editor/tsconfig.lib.json --noEmit reports 0 errors.
• tsc -p libs/block-editor/tsconfig.spec.json --noEmit reports 0 errors — the flags live in tsconfig.json, which the spec config extends, so spec errors are in scope.
• No new any. Anything unavoidable uses @ts-expect-error with a // TODO(#issue): note, never a blanket @ts-ignore.
• nx run block-editor:lint introduces no new findings.
• nx run block-editor:test does not regress — compared by failing-test name, not just count.
• The five dependents gain no new errors.

[!IMPORTANT]
typescript-strict-plugin / npx tsc-strict / @ts-strict-ignore are not used. Those were dropped early in the epic; the approach in force is documented in core-web/CLAUDE.md → TypeScript Strict Mode. The original ACs above have been corrected accordingly.

Measurement caveats

Five ways to get a green signal that checked nothing — all of them hit during this epic:

1. nx run <project>:test never type-checks. jest-preset-angular runs on ts-jest, which copies TypeScript's isolatedModules into its own transpile-only switch and skips building the language-service host diagnostics need. Passing tests are not evidence that specs compile.
2. TS2688 (a types entry that is not installed) aborts tsc before semantic checking — one error, no type checking at all.
3. TS6053 (a files entry pointing at a missing file) aborts the same way. A non-matching include glob is harmless; a missing files entry is fatal.
4. tsc -p does not check templates. Only a build does.
5. Check moduleResolution. node10 cannot resolve the @dotcms/* subpath exports, which inflated dot-plugins from 63 real errors to 733 phantom ones.

block-editor has no build target, so tsc -p on both configs is the acceptance test — there is no build to lean on.

Priority

Medium

Additional Context

One project in the bottom-up rollout tracked by epic #35932.

Read core-web/CLAUDE.md's TypeScript Strict Mode guidance and inspect libs/block-editor/tsconfig.json, tsconfig.lib.json, and tsconfig.spec.json. Run tsc -p on both project configs, then check nx run block-editor:lint and nx run block-editor:test, while verifying the five dependent projects gain no errors. Done means all six flags are in the project config, both checks pass without new any, and dependents remain clean.