MCP prompt names and arguments should be capped before embedding in prompt responses

[Widthdom]

Problem

prompts/get reads a client-supplied prompt name and argument strings, then embeds supported argument values directly into generated prompt text. Unsupported prompt names are also echoed in errors.

Evidence

• src/CodeIndex/Mcp/McpServer.cs: HandlePromptsGet reads params.name and params.arguments strings.
• src/CodeIndex/Mcp/McpServer.cs: prompt text interpolates path, scope, or symbol argument values.
• src/CodeIndex/Mcp/McpServer.cs: unknown prompt errors include the raw name.

Risk

Large prompt names or argument values can produce oversized prompt responses or diagnostics.

Suggested fix

Validate prompt name and argument string lengths before interpolation, and echo bounded display values in errors.

[Widthdom]

Starting work on this issue on branch fix-issue3116-3117-3118-3119-3120-3121-3122.