Bug description

If the port specified with --remote-auth-callback-port or in the UI is already in use, it seems ToolHive selects a random port instead. But, the authentication flow fails.

Steps to reproduce

Run a remote MCP server from the CLI or in the UI and specify a port that is already in use (specifically, one that is bound to 127.0.0.1) on your system.

thv run notion-remote --remote-auth-callback-port <SOME_IN_USE_PORT>

One way to reproduce is to run a local MCP using ToolHive, then try to use its port for the remote-auth-callback-port:

thv run fetch
thv list
# get its listening port...
thv run notion-remote --remote-auth-callback-port <FETCH_PORT>

Expected behavior

I would expect ToolHive to error out because the port is already in use.

IF it's going to "silently" select a different, random port, I'd expect a message in the output and for authentication to work using the random port.

Actual behavior

The notion authorization browser flow opens, shows the randomly-selected port in the callback URL, but the auth fails with an error:

failed to exchange code for token: oauth2: "invalid_grant" "Invalid redirect URI"

Environment (if relevant)

• OS/version: macOS, Docker Desktop
• ToolHive version: v0.3.7