feat(cdp): allow substring without length param

[MarconLP]

Problem

If you don't provide a length parameter, we should return the entire string starting from the provided start index.
https://posthoghelp.zendesk.com/agent/tickets/24971

Changes

• don't require the length param, and fallback to returning the entire string starting from the provided start index
• Check why the nodejs test is failing

Does this work well for both Cloud and self-hosted?

How did you test this code?

added test

[greptile-apps]

PR Summary

This PR modifies the substring function across Python and JavaScript implementations to make the length parameter optional, defaulting to returning the entire remaining string from the start index.

• Modified substring function in /common/hogvm/python/stl/__init__.py to handle optional length parameter
• Added test case in /common/hogvm/__tests__/stl.hog verifying substring behavior without length parameter
• Updated JavaScript STL implementation in /posthog/hogql/compiler/javascript_stl.py for consistency
• Needs investigation into failing Node.js test mentioned in PR description
• Added .cursorignore file to exclude .env from editor (seems unrelated to main changes)

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[greptile-apps]

logic: default length calculation s.length - start + 1 will return one character too many due to 1-based indexing

Suggested change

	"""function substring(s, start, length = s.length - start + 1) {
	"""function substring(s, start, length = s.length - start) {

[greptile-apps]

PR Summary

(updates since last review)

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[mariusandra]

I wouldn't put complex expressions in function arguments