fix: handle ValueError in render_tree() for incompatible Language versions

[clayarnoldg2m]

Summary

• Fixes Uncaught ValueError in __init__.py line 248 #4835 — TreeContext creation in render_tree() calls get_parser() which raises ValueError when the tree-sitter Language version is incompatible, crashing aider during repo map generation.
• Wraps TreeContext(...) with try/except ValueError, returning empty string on failure. This matches the existing defensive pattern in get_tags_raw() (lines 284-289 in the same file).

Test plan

• test_render_tree_language_version_error — mocks TreeContext to raise ValueError("Incompatible Language version"), verifies render_tree() returns ""
• test_get_repo_map_with_incompatible_language — same mock at get_repo_map() level, verifies no crash
• All 47 existing repomap tests pass

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