fix(js-sdk): pass sandbox headers to EnvdApiClient (#1175)

## Summary
- Fixes `Sandbox.create()` failing in self-hosted environments because
`sandboxHeaders` were not propagated to `EnvdApiClient`
- Spreads `sandboxHeaders` (`E2b-Sandbox-Id`, `E2b-Sandbox-Port`) into
the headers passed to the envd API client

Closes #1158
Based on #1159 by @ajuijas

## Test plan
- [ ] Verify sandbox creation works in self-hosted environments
- [ ] Verify sandbox headers are correctly passed to EnvdApiClient

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---------

Signed-off-by: ajuijas <ijas.ahmd.ap@gmail.com>
Co-authored-by: ajuijas <ijas.ahmd.ap@gmail.com>

Author: mishushakov

.changeset/tame-kiwis-listen.md

@@ -0,0 +1,5 @@
+---
+'e2b': patch
+---
+
+fix: pass sandbox headers to EnvdApiClient

packages/js-sdk/src/sandbox/index.ts

@@ -189,6 +189,7 @@ export class Sandbox extends SandboxApi {
         accessToken: this.envdAccessToken,
         headers: {
           'User-Agent': this.connectionConfig.headers?.['User-Agent'] ?? '',
+          ...sandboxHeaders,
           ...(this.envdAccessToken
             ? { 'X-Access-Token': this.envdAccessToken }
             : {}),