Graceful handling of EOF in MCP proxy server

[ayakut16]

Description

Similar to #13 but for proxy servers.

To reproduce:

~ mcp tools mcp proxy start                                
error: command error: exit status 1, stderr: Logging to /Users/alperen/.mcpt/logs/proxy.log
Proxy server started, waiting for requests...
Waiting for request...
Received request: tools/list (ID: 1)
Sending response
Waiting for request...
Error decoding request: EOF
2025/03/29 04:28:32 Error running proxy server: error decoding request: EOF

Changes:

• Graceful handling of EOF in MCP proxy server
• Print logs to stderr instead of stdout in main.go and proxy server

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

1. make build;./bin/mcp proxy tool add_op "Adds given numbers" "a:int,b:int" -e 'echo "totall is $a + $b = $(($a+$b))"'
2. ./bin/mcp shell ./bin/mcp proxy start
3. add_op {"a":1,"b":2}

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules