Salesforce Hosted MCP (sobject-all) — works in Production but "Authorization with the MCP server failed" in Sandbox ( ofid_2076830e5fdfad3a)

### What happened?

Summary
Connecting Claude's custom MCP connector to Salesforce Hosted MCP Server sobject-all. The same External Client App configuration works successfully against a Production org, but fails against a Sandbox org with an identical setup — Salesforce OAuth completes successfully (token issued, confirmed in Salesforce OAuth Usage), but Claude fails immediately after with "Authorization with the MCP server failed."
Environment

MCP Client: Claude (Web) — [precise if also tested on Desktop]
Org Type: Sandbox (failing) — Production (working, same ECA config)
MCP Server: sobject-all
Authentication: Authorization Code flow via Salesforce External Client App ("Claude")
App Authorization: All users can self-authorize
App Status: Enabled

### What did you expect to happen?

After completing OAuth and approving scopes on the Sandbox org, the connector should show as Connected and sobject-all tools should be available in conversations, consistent with the working Production connection using the same configuration.

### Steps to reproduce

- Activate sobject-all MCP server on a Production org; configure External Client App "Claude"; connect via Claude custom connector → works, tools available
- Replicate the exact same External Client App configuration and MCP server activation on a Sandbox org
- In Claude: Settings → Connectors → Add custom connector
- Enter the sandbox MCP server URL and Consumer Key in Advanced settings
- Click Connect → redirected to Salesforce OAuth
- Approve scopes (mcp_api, refresh_token/offline_access, api) — approval succeeds
- Redirected back to Claude → error shown

### Area

MCP Connector (adding/managing servers)

### MCP Server (if applicable)

sobject-all from Salesforces

### Error messages or logs

```shell
L'autorisation avec sClaude-objects-all a échoué. Vous pouvez vérifier vos identifiants et permissions. Si le problème persiste, partagez cette référence avec le support : « ofid_2076830e5fdfad3a »
```

### Additional context

This matches the pattern reported in #171 (Salesforce Hosted MCP sandbox specifically), #184, #251, #280 — multiple reports indicate the Salesforce Hosted MCP sandbox endpoint fails post-OAuth on Claude's side more consistently than production, with Anthropic support previously acknowledging similar cases as a broker-side issue rather than a Salesforce configuration problem.