Make CDP server more authoritative with respect to IDs

The TL;DR is that this commit enforces the use of correct IDs, introduces a
BrowserContext, and adds some CDP tests.

These are the ids we need to be aware of when talking about CDP:
- id
- browserContextId
- targetId
- sessionId
- loaderId
- frameId

The `id` is the only one that _should_ originate from the driver. It's attached
to most messages and it's how we maintain a request -> response flow: when
the server responds to a specific message, it echo's back the id from the
requested message. (As opposed to out-of-band events sent from the server which
won't have an `id`). When I say "id" from this point forward, I mean every id
except for this req->res id.

Every other id is created by the browser.

Prior to this commit, we didn't really check incoming ids from the driver. If
the driver said "attachToTarget" and included a targetId, we just assumed that
this was the current targetId. This was aided by the fact that we only used
hard-coded IDS. If _we_ only "create" a frameId of "FRAME-1", then it's tempting
to think the driver will only ever send a frameId of "FRAME-1".

The issue with this approach is that _if_ the browser and driver fall out of sync
and there's only ever 1 browserContextId, 1 sessionId and 1 frameId, it's not
impossible to imagine cases where we behave on the thing.

Imagine this flow:
- Driver asks for a new BrowserContext
- Browser says OK, your browserContextId is 1
- Driver, for whatever reason, says close browserContextId 2
- Browser says, OK, but it doesn't check the id and just closes the only
  BrowserContext it knows about (which is 1)

By both re-using the same hard-coded ids, and not verifying that the ids sent
from the client correspond to the correct ids, any issues are going to be hard
to debug.

Currently LOADER_ID and FRAEM_ID are still hard-coded. Baby steps.

Author: karlseguin

src/browser/browser.zig

@@ -98,12 +98,6 @@ pub const Browser = struct {
             self.session = null;
         }
     }
-
-    pub fn currentPage(self: *Browser) ?*Page {
-        if (self.session.page == null) return null;
-
-        return &self.session.page.?;
-    }
 };
 
 // Session is like a browser's tab.

src/cdp/browser.zig

@@ -33,7 +33,7 @@ pub fn processMessage(cmd: anytype) !void {
         setDownloadBehavior,
         getWindowForTarget,
         setWindowBounds,
-    }, cmd.action) orelse return error.UnknownMethod;
+    }, cmd.input.action) orelse return error.UnknownMethod;
 
     switch (action) {
         .getVersion => return getVersion(cmd),
@@ -88,7 +88,6 @@ test "cdp.browser: getVersion" {
 
     try ctx.processMessage(.{
         .id = 32,
-        .sessionID = "leto",
         .method = "Browser.getVersion",
     });
 
@@ -99,7 +98,7 @@ test "cdp.browser: getVersion" {
         .revision = REVISION,
         .userAgent = USER_AGENT,
         .jsVersion = JS_VERSION,
-    }, .{ .id = 32, .index = 0 });
+    }, .{ .id = 32, .index = 0, .session_id = null });
 }
 
 test "cdp.browser: getWindowForTarget" {
@@ -108,13 +107,12 @@ test "cdp.browser: getWindowForTarget" {
 
     try ctx.processMessage(.{
         .id = 33,
-        .sessionId = "leto",
         .method = "Browser.getWindowForTarget",
     });
 
     try ctx.expectSentCount(1);
     try ctx.expectSentResult(.{
         .windowId = DEV_TOOLS_WINDOW_ID,
         .bounds = .{ .windowState = "normal" },
-    }, .{ .id = 33, .index = 0, .session_id = "leto" });
+    }, .{ .id = 33, .index = 0, .session_id = null });
 }