chore: consolidate major dev-dependency upgrades (TypeScript 6, testing-library/react 16, jsdom 29)

[scotwells]

Summary

Consolidates three parked Dependabot major dev-dependency upgrades into a single change. Each was validated end-to-end locally against the current main; all three passed the full gauntlet, so all three are included here.

This supersedes the individual Dependabot PRs #6, #7, and #8 — they can be closed in favor of this one.

Included upgrades

Package	From	To	package.json	Verdict
typescript	~5.8.0	~6.0.3	root	Included (passes)
@testing-library/react	^14.0.0	^16.3.2	packages/app	Included (passes)
jsdom	^27.1.0	^29.1.1	root	Included (passes)

Validation

Ran against Node 22 / Yarn 4.13.0 after yarn install:

• yarn tsc:full — pass (full, non-incremental typecheck, --skipLibCheck false; exercises TypeScript 6.0.3 across the repo)
• CI=true yarn test:all — pass (2 suites, 7 tests; exercises @testing-library/react 16 rendering under the jsdom 29 test environment)
• yarn build:backend — pass
• yarn build:all — pass

yarn lint:all reports 2 pre-existing errors on main unrelated to these bumps: @backstage/theme is imported in packages/app but not declared in its package.json (@backstage/no-undeclared-imports). These are present on main before any change here and are not introduced or affected by these upgrades. Worth fixing separately.

Excluded upgrades

None. The concern going in was that TypeScript 6 might be incompatible with the pinned @backstage/cli (0.36.3), which would have forced excluding it. In practice the full typecheck, test suite, and both builds all pass with TypeScript 6.0.3, so it is kept.

Notes

• @backstage/cli still declares a jsdom: ^27.1.0 peer range, so yarn install emits a non-fatal peer-dependency notice for the jsdom 29 bump. The test environment runs correctly on jsdom 29 regardless; the notice is informational.
• These are all dev/test/build-time dependencies; there is no runtime/product-facing behavior change. The value is keeping the toolchain current (TS 6 language/typecheck, modern React Testing Library, current jsdom) as we operationalize the Backstage portal (see datum-cloud/infra#2967).