test(mcp): use generic example names in MCP server fixtures

[chihsuan]

Context

MCP-server tests hard-coded the internal context.a8c / context-a8c provider name as a generic placeholder, leaking org-internal naming into the public test suite.

TL;DR

Rename context.a8c / context-a8c in MCP-server test fixtures to generic example.server / example-server placeholders.

Summary

• Use example.server in agent_mcp_test.exs to keep the dotted/quoted-TOML-key branch of AgentMcp.codex_server_toml_block/2 covered.
• Use example-server consistently across codex_mcp_config_test.exs, workspace_and_config_test.exs, and app_server_test.exs for allowlist and rendered-config assertions.
• Update the example args path /srv/context-a8c/server.js to /srv/example-server/server.js to match.
• Tests-only change; no production code or behavior changes.

Alternatives

• Leave the internal name in test fixtures - rejected because it leaks Automattic-internal naming and the name is doing no specific work a generic placeholder couldn't do.
• Pick different generic names per file - rejected; one name per spelling is easier to skim across the suite.

Test Plan

• make all
• rg -n "a8c" test/ returns no remaining matches in renamed fixtures