Classify GITHUB_HOST case insensitively

[auraseb]

Summary

classifyHost now lowercases the host name before matching github.com and ghe.com, so a mixed case --gh-host / GITHUB_HOST is no longer treated as GitHub Enterprise Server.

Why

https://GitHub.com currently resolves to https://GitHub.com/api/v3/, and https://Contoso.GHE.com to https://Contoso.GHE.com/api/v3/, so every API call fails. Host names are case insensitive, and isLoopbackHost in the same file already lowercases before comparing.

Fixes #3524

What changed

• classifyHost (used by NewAPIHost and ParseHostType) compares a lowercased host name.
• Added TestParseAPIHost cases for a mixed case github.com host and a mixed case ghe.com subdomain.

MCP impact

• No tool or API changes

Prompts tested (tool changes only)

• Not applicable, no tool changes.

Security / limits

• No security or limits impact

Tool renaming

• I am not renaming tools as part of this PR

Lint & tests

• Linted locally with ./script/lint
• Tested locally with ./script/test

Docs

• Not needed