fix(runtime): honor refresh and retry in files write

[RedZapdos123]

Description:

Honor runtime refresh and retry in the multipart files write path.

Before this change, FilesystemAPI.Write resolved transport once from the bound sandbox and sent a single multipart /files request. As a result, a caller-provided WithRefresh hook could not surface a newly stamped runtime URL or TLS transport, and transient transport or HTTP 5xx failures ignored the configured WithRetry policy.

This PR keeps the existing outward /files error behavior, but refreshes the sandbox before each attempt, rebuilds the request from a cached multipart payload, and adds regression tests for the missing-runtime-URL refresh path and a transient 500 response.

Validation Tests run:

• go test ./pkg/utils/runtime -run 'TestWriteFileWithRuntime_RefreshResolvesRuntimeURL|TestWriteFileWithRuntime_RetriesTransientServerError' -count=1 -v
• go test ./pkg/utils/runtime -run 'TestWriteFileWithRuntime_(InputValidation|HTTPInteractions|TransportError|HonorsLargeArgsTimeout|RefreshResolvesRuntimeURL|RetriesTransientServerError)|TestTLSMode_CapabilityGroupsUseRuntimeTLS|TestTLSMode_CapabilityGroupsRejectInvalidBundle' -count=1

Notes:

• Scope is intentionally limited to FilesystemAPI.Write.
• ListDir and Remove are left for a follow up PR if needed.

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[codecov]

Codecov Report

❌ Patch coverage is 83.11688% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.55%. Comparing base (815a36d) to head (6227fc9).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
pkg/utils/runtime/filesystem.go	83.11%	9 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #904      +/-   ##
==========================================
+ Coverage   82.49%   82.55%   +0.06%     
==========================================
  Files         254      254              
  Lines       21585    21645      +60     
==========================================
+ Hits        17806    17869      +63     
+ Misses       3080     3076       -4     
- Partials      699      700       +1     

Flag	Coverage Δ
unittests	82.55% <83.11%> (+0.06%)	⬆️

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[RedZapdos123]

I think the current CI test failure is related to #791.

[RedZapdos123]

This E2E failure is separate from this PR.

sandbox (with sandbox-gateway) failed during quota pool startup: quota-small stayed 1/2 Running and AVAILABLE=0 before this PR's FilesystemAPI.Write path ran.

Tracked the issue in #908. Created the PR: #909 which adds a liveness probe so failed Code Interpreter startup restarts instead of remaining permanently unready.

The CI job lacks sandbox logs so the exact startup error is not proven there. Local reproduction matches this pod state.