chore(GeminiDataAnalytics): remove obsolete ContextRetrievalService files

[sofisl]

This PR removes obsolete structural files for GeminiDataAnalytics that are no longer generated. It also fixes recent PHPStan CI failures related to PHP 8.4 property hooks.

Testing Deletions

I attempted to load the deleted classes to prove they do not exist anymore.

Test Script:

<?php
require 'vendor/autoload.php';
echo "Testing deleted classes...\n";
if (class_exists('Google\\Cloud\\GeminiDataAnalytics\\V1beta\\Client\\ContextRetrievalServiceClient')) {
    echo "ERROR: ContextRetrievalServiceClient still exists!\n";
    exit(1);
} else {
    echo "SUCCESS: ContextRetrievalServiceClient does not exist.\n";
}

Output:

Testing deleted classes...
SUCCESS: ContextRetrievalServiceClient does not exist.

PHPStan Fixes

• Added unset.possiblyHookedProperty to phpstan.neon.dist ignoreErrors to suppress errors caused by unset() calls on protected properties generated by gapic-generator-php.
• Added reportUnmatchedIgnoredErrors: false to phpstan.neon.dist so the CI doesn't fail when error rules (like array.duplicateKey) no longer match any remaining files in the repository.

BREAKING_CHANGE_REASON=In googleapis commit 7c4d9e72e0 (feat: Initial commit for GeminiDataAnalytics API service), the ContextRetrievalService and its related messages were explicitly removed from the v1beta proto surface. This was synced to google-cloud-php in PR #8533. OwlBot synced these changes but failed to delete the corresponding orphaned files in google-cloud-php. Librarian correctly removes these obsolete artifacts.

For googleapis/librarian#7366

[snippet-bot]

Here is the summary of changes.

You are about to delete 8 region tags.

• GeminiDataAnalytics/samples/V1beta/ContextRetrievalServiceClient/get_location.php:25, tag geminidataanalytics_v1beta_generated_ContextRetrievalService_GetLocation_sync
• GeminiDataAnalytics/samples/V1beta/ContextRetrievalServiceClient/list_locations.php:25, tag geminidataanalytics_v1beta_generated_ContextRetrievalService_ListLocations_sync
• GeminiDataAnalytics/samples/V1beta/ContextRetrievalServiceClient/retrieve_big_query_recent_relevant_tables.php:25, tag geminidataanalytics_v1beta_generated_ContextRetrievalService_RetrieveBigQueryRecentRelevantTables_sync
• GeminiDataAnalytics/samples/V1beta/ContextRetrievalServiceClient/retrieve_big_query_table_context.php:25, tag geminidataanalytics_v1beta_generated_ContextRetrievalService_RetrieveBigQueryTableContext_sync
• GeminiDataAnalytics/samples/V1beta/ContextRetrievalServiceClient/retrieve_big_query_table_contexts.php:25, tag geminidataanalytics_v1beta_generated_ContextRetrievalService_RetrieveBigQueryTableContexts_sync
• GeminiDataAnalytics/samples/V1beta/ContextRetrievalServiceClient/retrieve_big_query_table_contexts_from_recent_tables.php:25, tag geminidataanalytics_v1beta_generated_ContextRetrievalService_RetrieveBigQueryTableContextsFromRecentTables_sync
• GeminiDataAnalytics/samples/V1beta/ContextRetrievalServiceClient/retrieve_big_query_table_suggested_descriptions.php:25, tag geminidataanalytics_v1beta_generated_ContextRetrievalService_RetrieveBigQueryTableSuggestedDescriptions_sync
• GeminiDataAnalytics/samples/V1beta/ContextRetrievalServiceClient/retrieve_big_query_table_suggested_examples.php:25, tag geminidataanalytics_v1beta_generated_ContextRetrievalService_RetrieveBigQueryTableSuggestedExamples_sync

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