Handle paginated MCP tool discovery - #27290

Summary

Fixes #26094.

• Follow every page returned by MCP tools/list during startup tool discovery instead of only exposing the first page.
• Add a seen-cursor guard so pathological pagination loops, including A -> B -> A, fail instead of spinning forever.
• Extend the streamable HTTP MCP regression server/test to cover bearer-token env vars, paginated tools/list, and initialized-session continuity.

Test plan

• just fmt
• just test -p codex-mcp with tools_list_cursor_guard_rejects_cursor_cycles
• just test -p codex-core streamable_http_bearer_env_var_discovers_paginated_tools
• just fix -p codex-mcp
• just fix -p codex-rmcp-client
• just fix -p codex-core

Adversarial review

• Real streamable HTTP path: the core regression starts the existing test_streamable_http_server binary over streamable HTTP rather than mocking the client in-process.
• Bearer token from env: the regression configures bearer_token_env_var and the server enforces authorization, but assertions and the test-only state endpoint expose only session ids, never token values.
• Session continuity: the test server records actual tools/list mcp-session-id headers and the integration test asserts at least two paginated calls, all nonempty, all equal.
• All paginated tools exposed: the model request must include the first-page echo tool and the second-page second_page_tool.
• A -> B -> A duplicate cursor guard: production pagination tracks all seen cursors with a HashSet, and codex-mcp has a focused unit test for that cycle.
• Stdio unaffected: the change is limited to the post-initialize tools/list pagination loop shared by MCP clients; stdio transport construction and launch paths are unchanged.
• Install-vs-connect wording: this fixes runtime discovery/connect behavior after a server is configured; it does not add server-specific installation assumptions or change auth/install UI copy.