fix(build): escape dynamic route segments in chunk names

[RezaRahemtola]

❓ Type of change

• 📖 Documentation (updates to the documentation, readme, or JSdoc annotations)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

getChunkName returns _routes/<path>.mjs for route handlers, and routeToFsPath converts dynamic segments like :id to [id]. Rollup rejects [id] as an invalid chunkFileNames placeholder, so building with the Rollup builder fails on nested dynamic routes like server/api/applications/[id]/context.get.ts.

It looks like Rolldown (the default builder) tolerates the brackets, which is why this slipped through.

This PR encodes brackets ([ and ]) as %5B/%5D in the chunk name so it's a valid chunkFileNames pattern.
This also avoids any collision with literal route segments named _id_ (the concern that closed the similar PR I found #4393).
Also updated the unit test and added a nested dynamic route case.

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.

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[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: 20ed5f63-7c38-47aa-968d-9701c7c4dea2

📥 Commits

Reviewing files that changed from the base of the PR and between 52abde8 and a1e3626.

📒 Files selected for processing (2)

• src/build/chunks.ts
• test/unit/chunks.test.ts

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📝 Walkthrough

Walkthrough

Changes

Route chunk encoding

Layer / File(s)	Summary
Encode dynamic route chunks src/build/chunks.ts, test/unit/chunks.test.ts	Route-derived chunk filenames percent-encode [ and ]. Tests cover direct and nested dynamic routes.
Estimated code review effort: 2 (Simple)	~10 minutes

Possibly related PRs

• nitrojs/nitro#4027: Both changes update route filesystem path handling in src/build/chunks.ts.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title follows Conventional Commits syntax and accurately describes the dynamic route chunk-name fix.
Description check	✅ Passed	The description clearly explains the Rollup failure, bracket encoding fix, test updates, and collision avoidance.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches 💡 1

🛠️ Fix failing CI checks 💡

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests

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