docs: fix plugin API documentation to reflect onStart/onEnd support (#27068)

## Summary
- Fixes the esbuild migration guide (`docs/bundler/esbuild.mdx`) which
incorrectly stated that `onStart`, `onEnd`, `onDispose`, and `resolve`
were all unimplemented. `onStart` and `onEnd` **are** implemented — only
`onDispose` and `resolve` remain unimplemented.
- Adds missing `onEnd()` documentation section to both
`docs/bundler/plugins.mdx` and `docs/runtime/plugins.mdx`, including
type signature, description, and usage examples.
- Adds `onEnd` to the type reference overview and lifecycle hooks list
in both plugin docs.

Fixes #27083

## Test plan
- Documentation-only change — no code changes.
- Verified the `onEnd` implementation exists in
`src/js/builtins/BundlerPlugin.ts` and matches the documented API.
- Verified `onStart` implementation exists and is fully functional.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Bot <claude-bot@bun.sh>
Co-authored-by: Claude <noreply@anthropic.com>

docs/bundler/esbuild.mdx

@@ -198,13 +198,16 @@ const myPlugin: BunPlugin = {
 };
 ```
 
-The builder object provides some methods for hooking into parts of the bundling process. Bun implements `onResolve` and `onLoad`; it does not yet implement the esbuild hooks `onStart`, `onEnd`, and `onDispose`, and `resolve` utilities. `initialOptions` is partially implemented, being read-only and only having a subset of esbuild's options; use `config` (same thing but with Bun's `BuildConfig` format) instead.
+The builder object provides some methods for hooking into parts of the bundling process. Bun implements `onStart`, `onEnd`, `onResolve`, and `onLoad`. It does not yet implement the esbuild hooks `onDispose` and `resolve`. `initialOptions` is partially implemented, being read-only and only having a subset of esbuild's options; use `config` (same thing but with Bun's `BuildConfig` format) instead.
 
 ```ts title="myPlugin.ts" icon="/icons/typescript.svg"
 import type { BunPlugin } from "bun";
 const myPlugin: BunPlugin = {
   name: "my-plugin",
   setup(builder) {
+    builder.onStart(() => {
+      /* called when the bundle starts */
+    });
     builder.onResolve(
       {
         /* onResolve.options */
@@ -225,6 +228,9 @@ const myPlugin: BunPlugin = {
         };
       },
     );
+    builder.onEnd(result => {
+      /* called when the bundle is complete */
+    });
   },
 };
 ```

docs/bundler/plugins.mdx

@@ -15,6 +15,7 @@ Plugins can register callbacks to be run at various points in the lifecycle of a
 - `onResolve()`: Run before a module is resolved
 - `onLoad()`: Run before a module is loaded
 - `onBeforeParse()`: Run zero-copy native addons in the parser thread before a file is parsed
+- `onEnd()`: Run after the bundle is complete
 
 ## Reference
 
@@ -39,6 +40,7 @@ type PluginBuilder = {
       exports?: Record<string, any>;
     },
   ) => void;
+  onEnd(callback: (result: BuildOutput) => void | Promise<void>): void;
   config: BuildConfig;
 };
 
@@ -423,3 +425,53 @@ This lifecycle callback is run immediately before a file is parsed by Bun's bund
 As input, it receives the file's contents and can optionally return new source code.
 
 <Info>This callback can be called from any thread and so the napi module implementation must be thread-safe.</Info>
+
+### onEnd
+
+```ts
+onEnd(callback: (result: BuildOutput) => void | Promise<void>): void;
+```
+
+Registers a callback to be run after the bundle is complete. The callback receives the [`BuildOutput`](/docs/bundler#outputs) object containing the build results, including output files and any build messages.
+
+```ts title="index.ts" icon="/icons/typescript.svg"
+const result = await Bun.build({
+  entrypoints: ["./app.ts"],
+  outdir: "./dist",
+  plugins: [
+    {
+      name: "onEnd example",
+      setup(build) {
+        build.onEnd(result => {
+          console.log(`Build completed with ${result.outputs.length} files`);
+          for (const log of result.logs) {
+            console.log(log);
+          }
+        });
+      },
+    },
+  ],
+});
+```
+
+The callback can return a `Promise`. The build output promise from `Bun.build()` will not resolve until all `onEnd()` callbacks have completed.
+
+```ts title="index.ts" icon="/icons/typescript.svg"
+const result = await Bun.build({
+  entrypoints: ["./app.ts"],
+  outdir: "./dist",
+  plugins: [
+    {
+      name: "Upload to S3",
+      setup(build) {
+        build.onEnd(async result => {
+          if (!result.success) return;
+          for (const output of result.outputs) {
+            await uploadToS3(output);
+          }
+        });
+      },
+    },
+  ],
+});
+```