refactor(browser): dispatch _get_cloud_provider through agent.browser_registry

Switches tools.browser_tool's cloud-provider lookup from the hardcoded
_PROVIDER_REGISTRY class-instantiation pattern to the
agent.browser_registry singleton registry that plugins self-populate.

Changes:

- tools/browser_tool.py top imports: pull BrowserProvider from
  agent.browser_provider (re-exported as CloudBrowserProvider for legacy
  callers) and the three provider classes from plugins/browser/<vendor>/.
  Legacy class names (BrowserbaseProvider, BrowserUseProvider, FirecrawlProvider)
  remain on tools.browser_tool as re-export shims so existing test patches
  (monkeypatch.setattr(browser_tool, 'BrowserUseProvider', ...)) keep working.

- _get_cloud_provider() now consults agent.browser_registry.get_provider()
  for explicit-config lookups. The auto-detect fallback still uses
  BrowserUseProvider() / BrowserbaseProvider() at the module level so the
  cache-policy test fixtures (which patch those names) keep driving the
  function. Test-time _PROVIDER_REGISTRY overrides are detected by class
  identity and routed through the legacy factory-call path.

- agent/browser_provider.py: BrowserProvider grows is_configured() and
  provider_name() as thin backward-compat aliases for the legacy
  CloudBrowserProvider API. Subclasses MUST implement is_available() and
  name; the aliases delegate. This keeps ~6 caller sites in browser_tool.py
  working without churning them.

- tests/tools/test_managed_browserbase_and_modal.py: _install_fake_tools_package
  grows stubs for agent.browser_provider / agent.browser_registry /
  plugins.browser.<vendor>.provider so the test's spec-loader path
  (sys.modules-reset + reload-tool-from-disk) can satisfy tools.browser_tool's
  top-level imports.

Verified: all 23 existing tests in test_browser_cloud_*.py +
test_managed_browserbase_and_modal.py still pass post-cutover.

The legacy tools/browser_providers/ directory is NOT yet deleted; several
tests still _load_tool_module() those files via spec_from_file_location.
The deletion + test-path updates land in a later commit.

agent/browser_provider.py

@@ -153,3 +153,23 @@ class BrowserProvider(abc.ABC):
             "tag": "",
             "env_vars": [],
         }
+
+    # ------------------------------------------------------------------
+    # Backward-compat shims for the legacy CloudBrowserProvider API
+    # ------------------------------------------------------------------
+    #
+    # The pre-PR-#25214 ABC exposed ``is_configured()`` and ``provider_name()``;
+    # ``tools.browser_tool`` has ~6 callers that still use those names. Rather
+    # than churn every callsite (and break out-of-tree downstream code that
+    # subclassed CloudBrowserProvider), we expose the old names as thin
+    # delegations to the new API. Subclasses MUST implement :meth:`is_available`
+    # and :attr:`name`; they may override ``is_configured`` / ``provider_name``
+    # for compatibility with the legacy ABC but it is not required.
+
+    def is_configured(self) -> bool:  # pragma: no cover - trivial delegation
+        """Backward-compat alias for :meth:`is_available`."""
+        return self.is_available()
+
+    def provider_name(self) -> str:  # pragma: no cover - trivial delegation
+        """Backward-compat alias returning :attr:`display_name`."""
+        return self.display_name