fix(worker-bundler): resolve package subpaths without an exports map

[mattzcarey]

Fixes #2466

• resolvePackage() tried the legacy main/module entry before the requested subpath, so fixture/internal resolved to fixture/index.js. This breaks packages like ajv-formats that import ajv/dist/compile/codegen.
• The legacy fallback now applies only to root imports. For packages without an exports map, subpaths fall through to extension/index resolution.
• A package that has an exports map keeps that boundary: a subpath it does not export stays unresolved instead of being read from disk.

-if (legacyEntry && typeof legacyEntry === "string") {
+if (!subpath && legacyEntry && typeof legacyEntry === "string") {
 ...
+if (subpath && pkg.exports !== undefined) {
+  return { path: specifier, external: true };
+}

Tests cover root, directory, file and scoped subpaths in transform-only and bundled mode, plus a subpath excluded by exports.

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🦋 Changeset detected

Latest commit: ad2ecf4

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@cloudflare/worker-bundler	Patch

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[agent-think]

✅ agents import sizes: no significant changes (2f3176b9 → ad2ecf44, workflow run)

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