Prevent wrangler pages dev from serving asset files outside of the build output directory

[GregBrimble]

Fixes https://cflare.co/24935

What this PR solves / how to test:

Ensures only files from within the build output directory can be served in wrangler pages dev.

Author has included the following, where applicable:

• Tests
• Changeset (Changeset guidelines)

Reviewer is to perform the following, as applicable:

• Checked for inclusion of relevant tests
• Checked for inclusion of a relevant changeset
• Checked for creation of associated docs updates
• Manually pulled down the changes and spot-tested

[changeset-bot]

🦋 Changeset detected

Latest commit: 2bd2283

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

This PR includes changesets to release 1 package

Name	Type
wrangler	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[github-actions]

A wrangler prerelease is available for testing. You can install this latest build in your project with:

npm install --save-dev https://prerelease-registry.devprod.cloudflare.dev/workers-sdk/runs/5325524919/npm-package-wrangler-3498

You can reference the automatically updated head of this PR with:

npm install --save-dev https://prerelease-registry.devprod.cloudflare.dev/workers-sdk/prs/3498/npm-package-wrangler-3498

Or you can use npx with this latest build directly:

npx https://prerelease-registry.devprod.cloudflare.dev/workers-sdk/runs/5325524919/npm-package-wrangler-3498 dev path/to/script.js

Additional artifacts:

npm install https://prerelease-registry.devprod.cloudflare.dev/workers-sdk/runs/5325524919/npm-package-cloudflare-pages-shared-3498

Note that these links will no longer work once the GitHub Actions artifact expires.

[codecov]

Codecov Report

Merging #3498 (2bd2283) into main (a72dc0a) will increase coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #3498      +/-   ##
==========================================
+ Coverage   75.18%   75.23%   +0.04%     
==========================================
  Files         183      183              
  Lines       11055    11064       +9     
  Branches     2904     2906       +2     
==========================================
+ Hits         8312     8324      +12     
+ Misses       2743     2740       -3     

see 4 files with indirect coverage changes