refactor: precompile host-app version regex and clarify test comments

- Use re.fullmatch() with module-level precompiled _HOST_APP_VERSION_RE
  instead of re.match() with anchors on every call
- Add comments clarifying behavior for invalid version (silently dropped)
  and invalid app name (host-app omitted entirely)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: Alon Yeshurun

src/fabric_cli/client/fab_api_client.py

@@ -15,6 +15,8 @@
 
 from fabric_cli.client.fab_api_types import ApiResponse
 from fabric_cli.core import fab_constant, fab_logger, fab_state_config
+
+_HOST_APP_VERSION_RE = re.compile(r"\d+(\.\d+){0,2}(-[a-zA-Z0-9\.-]+)?")
 from fabric_cli.core.fab_exceptions import (
     AzureAPIError,
     FabricAPIError,
@@ -338,9 +340,7 @@ def _get_host_app() -> str:
     host_app_version = os.environ.get(fab_constant.FAB_HOST_APP_VERSION_ENV_VAR)
 
     # validate host_app_version format is a valid version (e.g., 1.0.0)
-    if host_app_version and re.match(
-        r"^\d+(\.\d+){0,2}(-[a-zA-Z0-9\.-]+)?$", host_app_version
-    ):
+    if host_app_version and _HOST_APP_VERSION_RE.fullmatch(host_app_version):
         host_app += f"/{host_app_version}"
     return host_app
 

tests/test_core/test_fab_api_client.py

@@ -331,6 +331,7 @@ def __init__(self):
         ("Invalid-App", "1.0.0", ""),
         ("", None, ""),
         (None, None, ""),
+        # Invalid version format - host app is still included but version is silently dropped
         (
             "Fabric-AzureDevops-Extension",
             "1.2.0.4",  # Invalid format
@@ -422,6 +423,7 @@ def setup_default_private_links(mock_fab_set_state_config):
             "1.2.0",
             " host-app/fabric-azuredevops-extension/1.2.0",
         ),
+        # Invalid app name - host-app suffix is omitted entirely from User-Agent
         ("Invalid-App", "1.0.0", ""),
     ],
 )