fix(build): make the web build loadable outside Node

[niravanu]

dist/web could only be loaded by Node, so @google/adk could not be bundled for a browser, a worker, or any edge runtime. Two independent causes, both in core/build.js.

The createRequire banner was applied to the browser build. The shim exists so an ESM build can reach a CommonJS dependency, which is a Node concern, but the condition was format === 'esm' — and the web build is browser + esm. Every one of the 256 files in dist/web therefore opened with import {createRequire} from 'module', including leaf modules with no Node dependency at all. Bundlers targeting a browser resolve that as a missing package and stop.

The browser target included Safari 11, which predates async generators. esbuild lowered them, and its lowering of yield* super.method() emits __yieldStar(super.method()) in a scope where super is a syntax error, so models/apigee_llm.js did not parse. Safari 11 was never really supported — the library uses async generators throughout, and they are ES2018. The target only claimed it.

Measured before and after with wrangler, since Cloudflare Workers is the case that fails hardest:

before ✘ [ERROR] Unexpected "super"
dist/web/models/apigee_llm.js:142:25
after Total Upload: 4652.19 KiB / gzip: 795.49 KiB

And it runs, not merely builds: a Workflow with two FunctionNodes joining into a JoinNode, plus a RequestInput, constructed inside workerd and answering 200.

Adds a regression test. It asserts the two specific failures — no createRequire anywhere in dist/web, and every file parses — and deliberately does not assert that nothing imports a Node builtin, because several modules legitimately do (file_artifact_service, unsafe_local_code_executor, local_environment, skills/loader, a2a/*) and are only reached by importing them. The banner was different in kind: it was on everything. The test skips when dist/web has not been built, and it fails on the previous build and passes on this one.

The rest of the suite imports src, which is why neither of these was visible to it.

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[kalenkevich]

Totally make sense, thanks!