## 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
## Why
Remote HTTP MCP connections can source header values from the host environment,
so those variables must remain part of the connection identity even when the MCP
server runs in an executor environment.
## What changed
- Track `env_http_headers` variables for remote HTTP MCP connections.
- Continue tracking `bearer_token_env_var` only for local environments, where the
host resolves the token.
## Testing
Add coverage that verifies remote connections track host header variables without
tracking executor bearer-token variables, while local connections track both.
GitOrigin-RevId: 8fe175c54a101721a27b76495dcd298f0c2bc6a3
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