fix: prevent SQL injection in SnowflakeSearchTool and NL2SQLTool

[jmanico]

Summary

Fixes SQL injection vulnerabilities in SnowflakeSearchTool and NL2SQLTool reported in #4993.

SnowflakeSearchTool — the database and snowflake_schema parameters in _run() were interpolated directly into USE DATABASE / USE SCHEMA SQL statements via f-strings with no validation. This PR adds three layers of defense:

• Pydantic pattern validation on both SnowflakeSearchToolInput and SnowflakeConfig fields — rejects anything that isn't a valid Snowflake identifier before it reaches any code path
• Runtime _validate_identifier() check in _run() — defense-in-depth for callers that bypass Pydantic (e.g. direct method calls)
• Double-quoted identifiers in the SQL itself with embedded quote stripping — so even if both prior layers are somehow bypassed, Snowflake treats the value as a delimited identifier, not SQL syntax

NL2SQLTool — _fetch_all_available_columns() was using f-string interpolation to build a WHERE clause with the table name (flagged with # noqa: S608). Replaced with SQLAlchemy text() bind parameters, which is the standard parameterized query approach. Added a params argument to execute_sql() to support this.

Changes

• snowflake_search_tool.py: Added SNOWFLAKE_IDENTIFIER_PATTERN constant, _validate_identifier() helper, pattern validation on input/config fields, quoted identifiers in _run()
• nl2sql_tool.py: Switched _fetch_all_available_columns() to parameterized query, added params arg to execute_sql()
• snowflake_search_tool_test.py: Added comprehensive tests covering identifier validation, Pydantic-level rejection of injection payloads, defense-in-depth runtime checks, and quoted identifier verification

Test plan

• TestIdentifierValidation — validates _validate_identifier() accepts valid identifiers and rejects injection payloads (semicolons, quotes, spaces, parens, empty strings, leading numbers)
• TestInputSchemaValidation — confirms Pydantic rejects injection in database/schema fields
• TestConfigIdentifierValidation — confirms SnowflakeConfig rejects injection in database/schema
• test_run_rejects_injection_in_database / test_run_rejects_injection_in_schema — defense-in-depth tests at the _run() level
• test_run_uses_quoted_identifiers — verifies double-quoting in generated SQL

Closes #4993