fix(shared): missing port range validation and broken errors.As match bypass proxy 400 Bad Request

Problem

Edge client proxy request target parsing in packages/shared/pkg/proxy/host.go accepts invalid port values (port 0 or ports exceeding 65535), returning port = 0 or out-of-range ports to downstream proxy dialers. Furthermore, when InvalidSandboxPortError is returned, packages/shared/pkg/proxy/handler.go fails to match it in errors.As due to a pointer-type mismatch (*InvalidSandboxPortError vs value struct), causing invalid port requests to fall through to uncaught error handling rather than returning 400 Bad Request.

// Observed behavior when requesting port 0 or >65535:
// GetTargetFromRequest returns sandboxID, port=0, err=nil
// handler fails errors.As match and returns 500 / unhandled error instead of 400 Bad Request "Invalid sandbox port"

In addition, TestGetTargetFromRequest in packages/shared/pkg/proxy/host_test.go evaluated require.ErrorAs(t, err, &tt.wantErrIs) against a nil error interface, which matched any non-nil error and allowed error-type mismatches to pass silently.

Root Cause

1. parseHost and parseHeaders in host.go parsed ports using strconv.ParseUint without enforcing valid TCP port boundaries (1 <= port <= 65535).
2. handler.go declared var invalidPortErr *InvalidSandboxPortError as a pointer target for errors.As. Because host.go constructs InvalidSandboxPortError as a value struct (InvalidSandboxPortError{...}), errors.As evaluated to false due to double pointer indirection (**InvalidSandboxPortError).
3. host_test.go passed &tt.wantErrIs (*error) to require.ErrorAs, which executes errors.As(err, (*error)) and matches any non-nil error regardless of concrete error type.

Reproduction Steps

1. Call GetTargetFromRequest with host 0-sandboxid.e2b.app or HTTP header E2b-Sandbox-Port: 0.
2. GetTargetFromRequest returns sandboxID, 0, nil instead of InvalidSandboxPortError.
3. Call handler with a getDestination function returning InvalidSandboxPortError.
4. The handler fails to match errors.As(err, &invalidPortErr) and returns internal error instead of 400 Bad Request.

// Minimal unit test reproduction in handler:
h := handler(p, func(r *http.Request) (*pool.Destination, error) {
    return nil, InvalidSandboxPortError{Port: "invalid", wrapped: errors.New("invalid port")}
})
// Result: errors.As returns false when target is *InvalidSandboxPortError

Technical Context

• Files affected:
  • packages/shared/pkg/proxy/host.go
  • packages/shared/pkg/proxy/handler.go
  • packages/shared/pkg/proxy/host_test.go
  • packages/shared/pkg/proxy/proxy_test.go
• Subsystem: Edge Client Proxy Request Routing (packages/shared/pkg/proxy)
• Impact: Low - Medium

Proposed Changes

Start in packages/shared/pkg/proxy/host.go and read parseHost, parseHeaders, and their cases in host_test.go. Run the proxy package tests, then check handler.go and proxy_test.go for InvalidSandboxPortError handling. Done means ports 0 and above 65535 are rejected, errors.As identifies the error correctly, and invalid requests return 400 Bad Request.