Fix session parameters from connections.toml being ignored

[coder-jatin-s]

Fix #350

Previously, schemachange would ignore any session parameters defined in the connections.toml file because it was overwriting them with its own session parameters. This fix:

1. Preserves session parameters from connections.toml
2. Properly merges QUERY_TAG values (combining existing tags with schemachange's tag)
3. Maintains all other session parameters (e.g., QUOTED_IDENTIFIERS_IGNORE_CASE)

Example:
If your connections.toml has:
[STG]
session_parameters = {
QUOTED_IDENTIFIERS_IGNORE_CASE = false,
QUERY_TAG = "my_tag"
}

The session will now:

• Keep QUOTED_IDENTIFIERS_IGNORE_CASE = false
• Set QUERY_TAG = "my_tag;schemachange ;custom_tag" (if custom_tag provided)

Changes made:

• Modified SnowflakeSession.py to read session parameters from connections.toml first
• Added proper session parameter merging logic
• Added test coverage in test_session_parameters.py