[19/44] Enable TS strict mode in portlets-dot-experiments-data-access

Description

Part of the strict-mode rollout epic #35932. Enable TypeScript strict mode for the portlets-dot-experiments-data-access project.

• Dependency layer: 3 · Internal dependents: 2 · Rollout order: 19 / 44

Note: the original acceptance criteria referenced typescript-strict-plugin, npx tsc-strict, and // @ts-strict-ignore. That approach was dropped by the epic — the plugin was never installed. The ACs below reflect the approach in force, documented in core-web/CLAUDE.md → TypeScript Strict Mode.

Acceptance Criteria

• The six strict flags are present in the project's own tsconfig.json:
  forceConsistentCasingInFileNames, strict, noImplicitOverride, noPropertyAccessFromIndexSignature, noImplicitReturns, noFallthroughCasesInSwitch.
• tsc -p <projectRoot>/tsconfig.lib.json --noEmit reports 0 errors.
• tsc -p <projectRoot>/tsconfig.spec.json --noEmit reports 0 errors.
• All errors resolved with explicit types — no new any.
• nx run <project>:lint and :test pass.
• nx affected -t build,lint does not break dependents.
• The enforcement status is recorded — i.e. whether any CI gate actually verifies the strictness.

⚠️ Two things that make error counts untrustworthy here, both established earlier in this rollout:

• :test does not type-check. isolatedModules: true puts ts-jest in transpile-only mode (#35948), so passing tests say nothing about types. tsc -p is the acceptance test.
• A files entry pointing at a missing file (TS6053, #35947) or an uninstalled types entry (TS2688, #35944) aborts tsc before semantic checking. Confirm the compiler reached the code.

Priority

Medium

Additional Context

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

Read core-web/CLAUDE.md under TypeScript Strict Mode, then inspect the project's own tsconfig.json, tsconfig.lib.json, and tsconfig.spec.json. Run both tsc commands with --noEmit first, confirming they reach semantic checking, then use the listed Nx lint, test, and affected build commands. Done means all six flags are present, both compiles report zero errors without new any, checks pass, and enforcement status is recorded.