Fold keypair sample into existing update_connection_auth

update_connection_auth.py already covers exactly this workflow (change
a connection's auth_type + credentials via the update-connection
endpoint). The prior keypair-specific sample added no new code path --
only Snowflake-keypair context that belongs in the shared sample's
header.

Delete the dedicated keypair sample; expand update_connection_auth's
header comment to list common authentication_type values (including
auth-keypair) and to spell out the pre-saved-credential prerequisite
that applies to any embed_password=True conversion.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: jacalata

samples/update_connection_auth.py

@@ -1,3 +1,31 @@
+####
+# This script updates a single connection on a datasource or workbook to embed
+# credentials. It's a generic authentication-change helper: the same code path
+# works whether you're setting username+password, switching to OAuth, or moving
+# a Snowflake connection to keypair auth.
+#
+# Common authentication_type values:
+#   - "Username Password"      -- username + password auth
+#   - "oauth"                  -- OAuth
+#   - "auth-keypair"           -- Snowflake keypair auth (see prerequisite below)
+#   - "AD Service Principal"   -- Azure AD Service Principal
+#   - "sqlserver"              -- SQL Server username + password
+#
+# When embed_password=True the server binds the connection to a matching
+# pre-saved credential on the site (looked up by username, connection class,
+# and role). For keypair-auth conversions this means the Snowflake private key
+# MUST already be saved on the site under Site Settings -> Saved Credentials
+# for Data Sources before running this script. If not, the update writes the
+# new auth type into metadata but subsequent extract refreshes and connection
+# tests fail because no bound credential is found.
+#
+# Ability to change a connection's authentication type via this endpoint was
+# added in REST API v3.27 for datasource and workbook connections; flow
+# connections gained the same capability in v3.28.
+#
+# See: https://help.tableau.com/current/api/rest_api/en-us/REST/rest_api_ref_data_sources.htm#update_data_source_connection
+####
+
 import argparse
 import logging
 import tableauserverclient as TSC