fix(treeselect): emit onFilter when the user types in the filter input

[marmotz]

Description

p-treeSelect's onFilter output was never emitted: the component's own onFilterInput method was dead code, since the filter <input> is actually rendered inside the internal p-tree (which has its own onFilter output that nothing was listening to).
This fix wires (onFilter)="onFilterInput($event)" on the internal <p-tree>, and updates onFilterInput to accept the TreeFilterEvent emitted by p-tree, sync filterValue/filteredNodes, and re-emit it as TreeSelect's own onFilter event.
Before: typing in the filter box never fired onFilter on p-treeSelect.
After: onFilter fires with the current filter value and filteredValue, as documented.

Related issues

Fixes #256

Type of change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that changes the public API)
• Documentation only
• Refactor, test, or chore (no user-facing change)

Breaking changes

None

Test plan

• npm run build
• npm test
• npm run lint
• Verified in the demo app (if applicable)

Checklist

• Issue discussed or bug clearly described (link issue when applicable)
• Tests added or updated for behavioral changes
• Documentation updated (README, JSDoc, migration notes as needed)
• Public API changes documented; breaking changes called out
• CHANGELOG updated (if the repository maintains one and the change is user-facing)
• Commit messages follow Conventional Commits
• I agree to follow the OpenNG Foundation Code of Conduct

Additional context

Multiple users confirmed the same root cause and workaround in the issue comments (subscribing to the internal p-tree's onFilter and relaying it manually), this PR implements the equivalent fix directly in TreeSelect.