`html_rewriter_treats_esi_include_as_void_tag` compatibility_flag not taking effect in workerd

### What versions & operating system are you using?

```
System:
OS: macOS 26.1
CPU: (10) arm64 Apple M1 Max
Memory: 245.25 MB / 32.00 GB
Shell: 5.9 - /bin/zsh
Binaries:
Node: 22.17.1 - /Users/timkelty/.nvm/versions/node/v22.17.1/bin/node
npm: 10.9.2 - /Users/timkelty/.nvm/versions/node/v22.17.1/bin/npm
pnpm: 8.15.4 - /opt/homebrew/bin/pnpm
npmPackages:
@cloudflare/vitest-pool-workers: ^0.10.6 => 0.10.7
wrangler: ^4.47.0 => 4.48.0
```

### Please provide a link to a minimal reproduction

https://github.com/timkelty/cloudflare-esi-compat-flag-bug

### Describe the Bug

# Compatibility flags from wrangler.jsonc not applied in vitest-pool-workers test environment

## Description

The `html_rewriter_treats_esi_include_as_void_tag` compatibility flag specified in `wrangler.jsonc` is not being applied when running tests with `@cloudflare/vitest-pool-workers`. This causes HTMLRewriter to throw a `TypeError: Parser error: Unsupported pseudo-class or pseudo-element in selector` when trying to use `esi:include` as an element selector.

See reprocase: https://github.com/timkelty/cloudflare-esi-compat-flag-bug