## Why
Older executors do not support resolving HTTP header values from their own
environment. Sending environment-backed bearer tokens to them can prevent MCP
servers from starting.
## What changed
- Advertise executor support for environment-backed HTTP headers with the
`httpHeaderEnvVars` capability.
- Delegate bearer-token resolution only when the selected executor advertises
that capability; otherwise, resolve tokens available in the host environment.
- Exclude executor-owned MCP servers that require environment-backed bearer
tokens when the executor cannot resolve them, while retaining compatible
host-owned and executor-owned servers.
## Testing
Added protocol and environment-variable coverage plus an end-to-end test that
simulates an older executor and verifies compatible MCP servers remain usable.
GitOrigin-RevId: 43545926f6f7e0f3b1091eb29d34647f6372bc4a
8edb95f274