Add expand limit to connection

[kmcginnes]

Description

Adds a global node expansion limit to the connection.

• Default is off for new and existing connections
• Updated documentation for new environment value GRAPH_EXP_NODE_EXPANSION_LIMIT
• Ensure fetch neighbors and neighbor counts queries respect the limit
• Updated tests

Validation

• Tested limits respected across query engines
• Tested queries are refetched when limit changes
• Tested new and existing connections function as expected

Related Issues

• Resolves [Task] Add expansion limit option per connection #442

Check List

• I confirm that my contribution is made under the terms of the Apache 2.0
  license.
• I have run pnpm checks to ensure code compiles and meets standards.
• I have run pnpm test to check if all tests are passing.
• I have covered new added functionality with unit tests if necessary.
• I have added an entry in the Changelog.md.