chore(deps): update dependency go to v1.25.5 (#2003)

This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
| [go](https://go.dev/)
([source](https://redirect.github.com/golang/go)) | toolchain | patch |
`1.25.3` -> `1.25.5` |

---

### Release Notes

<details>
<summary>golang/go (go)</summary>

###
[`v1.25.5`](https://redirect.github.com/golang/go/compare/go1.25.4...go1.25.5)

###
[`v1.25.4`](https://redirect.github.com/golang/go/compare/go1.25.3...go1.25.4)

</details>

---

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rebase/retry checkbox.

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---

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---

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Co-authored-by: Averi Kitsch <akitsch@google.com>

go.mod

@@ -2,7 +2,7 @@ module github.com/googleapis/genai-toolbox
 
 go 1.24.7
 
-toolchain go1.25.3
+toolchain go1.25.5
 
 require (
 	cloud.google.com/go/alloydbconn v1.15.5