fix(mdx): Handle read-only filesystems during MDX compilation (#15453)

More research needed. It appears like we are generating files (or at
least a directory) at build-time in an immutable vercel lambda task. We
need to figure out why, then either move it to /tmp or prevent it all
together, which is what this PR is geared towards right now.

-----

## DESCRIBE YOUR PR
*Tell us what you're changing and why. If your PR **resolves an issue**,
please link it so it closes automatically.*

Fixes
[DOCS-915](https://sentry.io/organizations/sentry/issues/6544953280/).
The issue was that: esbuild failed to create the image output directory
in the read-only Lambda environment because `options.write: true`
triggered an unprotected internal `mkdir` call.

- Prevents esbuild from automatically writing files during MDX
compilation.
- Manually writes output files from esbuild only during build time (when
filesystem is writable).
- Gracefully handles read-only filesystems (e.g., Lambda runtime) by
skipping file writing if it fails, assuming files already exist from
build time.


This fix was generated by Seer in Sentry, triggered by Cody De Arkland.
👁️ Run ID: 2524763

Not quite right? [Click here to continue debugging with
Seer.](https://sentry.io/organizations/sentry/issues/6544953280/?seerDrawer=true)

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Co-authored-by: seer-by-sentry[bot] <157164994+seer-by-sentry[bot]@users.noreply.github.com>
Co-authored-by: Kyle a.k.a. TechSquidTV <git@techsquidtv.com>
Co-authored-by: getsantry[bot] <66042841+getsantry[bot]@users.noreply.github.com>

Author: 3 people

src/mdx.ts

@@ -683,8 +683,9 @@ export async function getFileBySlug(slug: string): Promise<SlugFile> {
       // for this specific slug easily
       options.outdir = assetsCacheDir || outdir;
 
-      // Set write to true so that esbuild will output the files.
-      options.write = true;
+      // Set write to false to prevent esbuild from writing files automatically.
+      // We'll handle writing manually to gracefully handle read-only filesystems (e.g., Lambda runtime)
+      options.write = !!process.env.CI;
 
       return options;
     },
@@ -694,6 +695,10 @@ export async function getFileBySlug(slug: string): Promise<SlugFile> {
     throw e;
   });
 
+  // Manually write output files from esbuild when available
+  // This only happens during build time (when filesystem is writable)
+  // At runtime (Lambda), files already exist from build time
+
   const {code, frontmatter} = result;
 
   let mergedFrontmatter = frontmatter;