fix: do not normalize the user provided endpoint

[wzxxing]

Description of changes:

Users of aws-mcp-proxy can connect to their private MCP servers deployed to bedrock agentcore runtime. They get back a URL like below:

agent_arn = "arn:aws:bedrock-agentcore:us-west-2:<account-id>:runtime/private-mcp-server-id"
encoded_arn = agent_arn.replace(':', '%3A').replace('/', '%2F')
mcp_url = f"https://bedrock-agentcore.us-west-2.amazonaws.com/runtimes/{encoded_arn}/invocations?qualifier=DEFAULT"

Check https://docs.aws.amazon.com/bedrock-agentcore/latest/devguide/runtime-mcp.html for details.

In this case, the MCP endpoint url should not be normalized.

From users perspective, this also does not make sense. /mcp is just a convention, not a requirement for streamable-http servers.

Testing

Successfully connected to my remote MCP on agentcore runtime.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.