fix(onboarding): Preserve user-entered project slug during platform selection

[sentry]

This PR addresses a bug where a user-entered project slug would be overwritten when a platform was selected during the project creation onboarding flow.

Problem:
Previously, the logic in handlePlatformChange attempted to determine if the project name was user-modified by comparing formData.projectName with formData.platform?.key. This heuristic was flawed:

1. If a user typed a project name that happened to match the currently selected platform's key, the system would incorrectly assume it was auto-generated and overwrite it when a different platform was selected.
2. The useEffect that synced initialData to formData could inadvertently reset projectName if initialData re-evaluated (e.g., due to async team loading), clobbering user input.
3. PlatformPicker's debounceSearch captures setPlatform from the initial render via useRef. When the user typed in the filter bar to auto-select a platform, the stale closure would call the first-render handlePlatformChange — where formData.projectName was still '' — wiping the user's slug even though the hasUserModifiedProjectName ref was correctly set to true.

Solution:

1. Introduced a useRef, hasUserModifiedProjectName, to explicitly track whether the user has manually typed into the project slug input. This provides a reliable signal that the user intends to keep their custom name.
2. The onChange handler now sets hasUserModifiedProjectName.current = true when the user types, and resets it to false if the field is cleared, allowing platform auto-fill to take over again.
3. handlePlatformChange now uses a functional setFormData(prev => ...) updater instead of closing over formData.projectName. This ensures prev.projectName always reflects current state, even when the debounce fires with a stale closure.
4. The useEffect that syncs initialData now skips projectName when hasUserModifiedProjectName.current is true, preventing async re-evaluation from clobbering user input.

Tests:
Added new test cases to createProject.spec.tsx to cover:

• Ensuring a user-entered name (even one matching a platform key) is preserved when switching platforms.
• Ensuring a user-entered slug is preserved when the filter bar auto-selects a platform via debounce.
• Verifying that clearing the project name input allows platform selection to re-populate the field.

Ref NEW-769

[linear]

NEW-769 Onboarding renames project to “other” after selecting platform

[cursor]

Cursor Bugbot has reviewed your changes and found 2 potential issues.

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[priscilawebdev]

I tested the changes and everything looks good. I found two additional bugs and have already pushed fixes to this PR.