fix: align claude oauth request shape

[harshav167]

Summary

• align Claude Code OAuth requests to Claude CLI shape (beta=true, claude-cli user-agent)
• normalize headers and remove fine-grained tool streaming beta for OAuth
• keep tool-name prefixing from PR fix: claude oauth by renaming tools on in/out anomalyco/opencode-anthropic-auth#10 (oc_)

Why this over PR anomalyco#10

PR anomalyco#10 prefixes tool names, which helps tool-call routing, but the OAuth rejection happens before tool calls are processed. The 400 error is returned from /v1/messages itself. This change fixes the request signature mismatch (endpoint query + user-agent + beta list) so Claude Code OAuth tokens are accepted.

Notes

• This is scoped to OAuth requests only; API key behavior is unchanged.

Test plan

• Verified locally by routing OpenCode through ccflare and confirming POST /v1/messages?beta=true with user-agent: claude-cli/... and OAuth beta list; request succeeds.