fix: correct gvisor replace directive to match module path (#26822) - #27137

## Summary

Fixes the gvisor `replace` directive in `go.mod` to target the correct
module path.

## Problem

PR #23055 added a replace directive to use the coder/gvisor fork (which
fixes an integer overflow causing `panic: length < 0` crashes). However,
the directive targeted the wrong module path:

```
replace gvisor.dev => github.com/coder/gvisor v0.0.0-20260313164934-7a658db7b714
```

The actual module path declared in gvisor's `go.mod` is
`gvisor.dev/gvisor`, not `gvisor.dev`. Go module replace directives
require an exact module path match, so the previous directive was a
no-op and the patched fork was never used.

## Fix

```diff
-replace gvisor.dev => github.com/coder/gvisor v0.0.0-20260313164934-7a658db7b714
+replace gvisor.dev/gvisor => github.com/coder/gvisor v0.0.0-20260313164934-7a658db7b714
```

## Validation

Verified locally with `go list -m`:

**Before (no-op replace):**
```
$ go list -m gvisor.dev/gvisor
gvisor.dev/gvisor v0.0.0-20240509041132-65b30f7869dc
```

**After (correct replace):**
```
$ go list -m gvisor.dev/gvisor
gvisor.dev/gvisor v0.0.0-20240509041132-65b30f7869dc => github.com/coder/gvisor v0.0.0-20260313164934-7a658db7b714
```

The `=>` confirms the fork is now applied.

Fixes #20885

---

<details>
<summary>Investigation context</summary>

- The coder/gvisor fork (commit `7a658db7b714`) declares `module
gvisor.dev/gvisor` in its go.mod
- Customer runtime stack traces show
`gvisor.dev/gvisor@v0.0.0-20240509041132-65b30f7869dc` (unpatched
upstream), confirming the fork was not applied
- The crash is `panic: length < 0` in
`gvisor.dev/gvisor/pkg/tcpip/transport/tcp.(*sender).splitSeg`
- Related Linear ticket: ENT-118

</details>

---
*Generated by [Coder Agents](https://coder.com/agents) on behalf of
@denisra*

(cherry picked from commit f77d006)