chore(flow-server): remove the dead GWT client-engine bootstrap emission

[totally-not-ai]

Summary

BootstrapHandler emitted a client.nocache.js <script> and computed the GWT client-engine URL, but the live client loads via Flow.ts -> FlowClient, so this path is dead once GWT is gone. Removes getClientEngineUrl/getClientEngine, the CLIENT_ENGINE_NOCACHE_FILE constant and its emission, and the matching mock in MockServletServiceSessionSetup.

Verified: no callers of the removed API anywhere in the repo. Placed after the GWT removal so the emission is unambiguously dead.

Stacked on #24954 (base flow-client-ts-remove-gwt) — merge after it. Non-draft; please review before merging.

[totally-not-ai]

Review — remove dead GWT client-engine bootstrap emission

Correct and clean. BootstrapHandler no longer emits the <script src=".../client.nocache.js"> element (head.appendChild(createJavaScriptElement(getClientEngineUrl(context)))), and the now-unreachable helpers getClientEngineUrl, getResourceProvider, getClientEngine, plus the CLIENT_ENGINE_NOCACHE_FILE constant are removed. After the TS cutover the client loads via the frontend FlowClient module, so this GWT loader script would 404 — removing it is the right move.

Verified no fallout in BootstrapHandler.java:

• The dropped imports Properties, Lookup, ResourceProvider have 0 remaining uses (no dangling/unused imports → checkstyle-clean).
• InputStream and IOException are correctly retained — still used elsewhere in the class (e.g. readResource, the request handlers).

The paired MockServletServiceSessionSetup change removes the matching compile.properties mock stub, and its ByteArrayInputStream/StandardCharsets/IOException imports are still used by other stubs, so nothing is left dangling.

Minor (optional, for consistency)

An identical now-dead getClientResourceAsStream(".../compile.properties") mock stub still exists in flow-server/src/test/.../WebComponentBootstrapHandlerViteTest.java (initLookup). WebComponentBootstrapHandler (main) doesn't read the client engine, so that stub is unused — consider removing it alongside the MockServletServiceSessionSetup one (and note that CLIENT_ENGINE_PATH becomes referenced only by that dead stub afterwards). Not blocking.

LGTM.