command: Respect smart sync override declaratively by default

Introduce a `RESPECT_SMART_SYNC_OVERRIDE` class attribute to the base
`Command` class, defaulting to `True`. This allows subcommands to
automatically respect the smart sync override manifest if it exists.

The override is applied in `CommonValidateOptions` before any
subcommand-specific validation or execution occurs. The `sync` and
`init` commands explicitly opt out.

This ensures all workspace-aware subcommands consistently align with the
active smart sync override manifest. It also fixes a bug in
multi-manifest setups where running a command from a submanifest would
not apply the override to the outer manifest, causing inconsistency when
resolving projects across all manifests.

Bug: 279204331
Change-Id: I9426e90a13a77ce6bd94b4a82efda4d485cbe116
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/585081
Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Gavin Mak <gavinmak@google.com>

tests/test_subcmds_gc.py

@@ -24,7 +24,7 @@ class GcCommand(unittest.TestCase):
     """Tests for gc command."""
 
     def setUp(self):
-        self.cmd = gc.Gc()
+        self.cmd = gc.Gc(manifest=mock.MagicMock())
         self.opt, self.args = self.cmd.OptionParser.parse_args([])
         self.opt.this_manifest_only = False
         self.opt.repack = False