fix(downloads): don't address the browser's implicit context by id

[route]

Closes #546.

The bug

Downloads#set_behavior sent browserContextId: @page.context_id unconditionally. For a page in the browser's implicit context — the one Chrome puts its startup window in — that id isn't addressable, so the first page died with:

Ferrum::BrowserError: Failed to find browser context for id 97B71EC807EDF5868B937479FC2E4324
  lib/ferrum/client.rb:367 raise_browser_error
  lib/ferrum/page.rb:448   Ferrum::Page#command

It reproduces on main against any browser that came up with its own startup window (a Chrome in a sidecar container, browserless) when :save_path is set, since prepare_page configures the download behavior for every page. It is not flatten-specific — flatten: false fails identically — and not version-specific either, Chrome 152 fails the same way as the 119 in the report. Same root cause as #578/#627, which fixed it for Target.createTarget only.

The fix

Target#context_id, and with it Page#context_id, is nil for the implicit context, so a caller scoping a command to a browser context can tell it apart from the id alone, without reaching for the context object:

options = { browserContextId: @page.context_id } if @page.context_id
@page.command("Browser.setDownloadBehavior", **Hash(options), ...)

Context#create_target and Contexts#dispose keep the implicit? checks they already had, so all three CDP calls that take a browserContextId are covered. Those three are the only ones in the codebase — everything else is target- or session-scoped.

Two alternatives that don't work, both checked against a live browser:

• Always omitting browserContextId: downloads in a context we created come back canceled, the behavior lands on the default context instead.
• Setting the behavior once per context from the browser session: files save, but Browser.downloadWillBegin/downloadProgress are then delivered to the browser session and page.downloads.files/#wait track nothing. The command has to stay on the page's session.

Cuprite builds its own pages with Page.new(target.client, context_id: target.context_id, ...), so it picks this up unchanged.

Verification

• New spec in spec/downloads_spec.rb, using with_external_browser(incognito: false) for a browser with a startup window. It fails with Failed to find browser context for id without the fix.
• Full suite: 614 examples, 0 failures.
• Manually, against a startup-window Chrome in both flatten modes: the download completes, the file is saved and stays tracked on the page, while a context created by Ferrum in the same browser still saves to its own save_path.
• rbs validate passes. Page#context_id is String? now, since Target is what passes it and it can be nil.