bundle option breaks handler resolution in dev mode #6253

Use Case

I use a custom build pipeline (tsdown/rolldown) instead of SST's built-in esbuild because my project includes native binaries that require special bundling treatment (externals, copy steps). The build outputs to a dist/ directory which I reference via the bundle option.

This works perfectly with sst deploy, but breaks in sst dev.

Configuration

new sst.aws.Function("MyApi", {
  bundle: "./dist",
  handler: "lambda.handler",
});

Error

TypeError [ERR_INVALID_ARG_TYPE]: The "path" argument must be of type string. Received undefined
    at Object.pathToFileURL (node:url:1024:3)
    at .sst/platform/dist/nodejs-runtime/index.js:33:24

Cause

SST's dev mode nodejs-runtime tries to resolve the handler path using logic that expects source files relative to the project root. When bundle is provided, this resolution fails and returns undefined, which then crashes pathToFileURL().

Deploy mode works because it just zips the bundle directory without path resolution.

Workaround

new sst.aws.Function("MyApi", {
  ...($dev
    ? { handler: "./src/lambda.handler" }
    : { bundle: "./dist", handler: "lambda.handler" }),
});

Suggested Fix

When bundle is set, resolve handler paths relative to the bundle directory, not the project root.