feat: add 'methodNotAllowed' handler for '405 Method Not Allowed' responses

[silvioprog]

Summary

Adds an opt-in app.methodNotAllowed() handler that returns 405 Method Not Allowed responses when a path exists but the HTTP method is not supported. This improves HTTP semantics by properly distinguishing between routes that don't exist (404) and routes that exist but don't support the requested method (405).

Changes

• Added MethodNotAllowedHandler and MethodNotAllowedResponse types
• Added methodNotAllowed() method to HonoBase class
• Implemented #findAllowedMethods() helper to detect allowed methods for a path
• Updated #dispatch() to check for method not allowed when no route matches
• Added comprehensive test suite covering various scenarios
• Exported types from main index

Behavior

• Opt-in: Feature is disabled by default (returns 404 as before)
• When enabled: Returns 405 with Allow header listing permitted methods
• Backward compatible: Existing behavior unchanged when handler is not set

Example Usage

const app = new Hono()

app.get('/hello', (c) => c.text('Hello'))

// Enable method not allowed handling
app.methodNotAllowed((c, allowedMethods) => {
  return c.text('405 Method Not Allowed', 405, {
    Allow: allowedMethods.join(', ').toUpperCase(),
  })
})

// POST /hello will now return 405 with Allow: GET
// GET /hello will return 200
// GET /nonexistent will return 404 (path doesn't exist)

Testing

• Added tests
• All tests pass (bun run test)
• Code formatted (bun run format:fix)
• Linting passes (bun run lint:fix)
• TSDoc added to public APIs

Related

Closes #4633