This commit adds a new x402-protected chat endpoint at /api/x402/chat
with a price of $0.01 per request. All traffic to /api/chat now flows
through the x402 endpoint using credit deductions and USDC payments.

Changes:
- Add CHAT_PRICE constant ($0.01) to lib/const.ts
- Configure x402 middleware to protect POST /api/x402/chat
- Create /api/x402/chat/route.ts endpoint handler
- Add handleChatStreamX402 for x402-specific streaming
- Add validateChatRequestX402 for x402 body validation (accountId required)
- Add validateChatAuth for auth-only validation
- Add fetchWithPaymentStream for POST requests with streaming
- Add x402Chat wrapper function for calling x402 endpoint
- Update /api/chat to route through x402 with credit deduction
- Add comprehensive tests for all new functions

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Extract the shared streaming logic into streamChatResponse function
that both handleChatStream and handleChatStreamX402 can use. This
eliminates code duplication and follows the KISS principle.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Since /api/chat now routes through x402, handleChatStream is no longer
used. Remove it and its tests to follow YAGNI principle.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

The middleware matcher was running on all /api/* routes, which caused
CORS preflight (OPTIONS) requests to /api/chat to be intercepted and
redirected by the x402 middleware. Now the middleware only runs on
/api/x402/* routes where x402 payment protection is needed.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

The x402 internal response already contains CORS headers, and we were
adding our own on top, resulting in 'Access-Control-Allow-Origin: *, *'.
Now we filter out any access-control-* headers from the internal
response before adding our own CORS headers.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

loadAccount was hardcoded to use IMAGE_GENERATE_PRICE ($0.15) for all
transfers. Now it accepts a price parameter so chat uses CHAT_PRICE
($0.01) and image generation uses IMAGE_GENERATE_PRICE ($0.15).

Changes:
- Update loadAccount to accept price parameter
- Update fetchWithPayment to pass IMAGE_GENERATE_PRICE
- Update fetchWithPaymentStream to pass the price parameter
- Add tests for loadAccount
- Update fetchWithPaymentStream tests

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

When streaming responses go through proxy middleware, they can be
buffered/compressed causing glitchy streaming. Adding Content-Encoding:
none header prevents this.

See: https://ai-sdk.dev/docs/troubleshooting/streaming-not-working-when-proxied

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>