Custom connector to docsmcp.googleapis.com fails with "caller does not have permission" on every call (same bug class as #218)

### MCP server URL or name

https://docsmcp.googleapis.com/mcp/v1

### How are you connecting to this server?

From a custom connector URL I entered manually

### Where does the issue occur?

During a conversation (after initial auth)

### When did you last reproduce this?

2026-08-16 10:04

### Browser and OS

Claude cowork desktop

### Describe the issue

Connector authenticates successfully — claude.ai shows "Connected" — but every tool call made afterward mid-conversation fails with "The caller does not have permission." Reproduced repeatedly, most recently 2026-08-16 via read_doc on a document the authenticated account owns and has edit access to.

Ruled out before concluding this is a platform bug (same class as #218):
- Account mismatch: confirmed the Google account used for the OAuth grant, the GCP console project, and the target file's owner are all the same account.
- Missing scopes: Google's own setup guide for this MCP server requires drive.readonly, drive.file, documents.readonly, documents. Initially only 3 of 4 were configured; added the missing .readonly scope so all were present and visible on the consent screen during authorization.
- Stale/cached token: fully revoked access server-side via myaccount.google.com/permissions → app → "Delete all" (not just claude.ai's Disconnect button), then reconnected from scratch with a brand-new consent screen showing all scopes.

Error was identical before and after all of the above. A separate, older custom connector to Google's Drive MCP server, authorized under the same account, works correctly on the same files — so this is specific to the Docs MCP endpoint, not a general OAuth problem. Steps to reproduce: create a GCP project, enable docs.googleapis.com and docsmcp.googleapis.com, configure OAuth consent with the 4 scopes above, add https://docsmcp.googleapis.com/mcp/v1 as a custom connector, complete auth, then call read_doc on any owned file.

Related: #218 (Google People API, same error message and root cause theory) and #42 (referenced in #218 as an earlier instance of the same scope-forwarding bug class).

### Error details

```shell
The caller does not have permission

No pop-up error or error ID was shown in the claude.ai UI — this is the raw error text returned by the tool call itself when the connector is used (e.g. via read_doc).
```

### Expected behavior

Tool call succeeds and returns the document's content, matching the access already granted on the OAuth consent screen.

### Additional context

Impact: Google's official Docs remote MCP server is currently unusable from claude.ai via custom connector, blocking any workflow that needs Claude to read or edit live Google Docs content directly. Companion issue for the Sheets MCP endpoint, which fails identically: #861