Fix/blob text renders as hex

[sykesab]

Summary

• Decode bytes / bytearray / memoryview result values as UTF-8 before building the Arrow backend; fall back to 0x… hex only when the bytes are not valid UTF-8. Text stored in BLOB columns (for example JSON in a MySQL mediumblob) renders as text again, and the V value viewer detects JSON and opens tree mode.
• Wrap the v cell-preview tooltip value in rich.text.Text so Textual does not parse cell content as Rich markup. Previously a value containing a closing-tag shape such as "[/api/v1]" raised MarkupError from the hover timer and exited the app.

Fixes #328.

Changes

• sqlit/shared/ui/widgets_tables.py: normalize_arrow_value tries raw.decode("utf-8") and returns hex on UnicodeDecodeError. Invalid UTF-8 keeps the behaviour introduced for TUI crashes on raw binary result columns with UnicodeDecodeError after v1.6.3 upgrade #322.
• sqlit/domains/results/ui/mixins/results.py: _show_cell_tooltip assigns Text(tooltip_value) instead of the bare string.
• tests/ui/test_uuid_fastdatatable.py:
  • UTF-8 JSON bytes render as the decoded string, parametrized over bytes, bytearray, and memoryview.
  • Incrementally added UTF-8 bytes via add_rows render as the decoded string.
  • action_view_cell on a JSON blob containing [/api/v1] sets a Text tooltip with the decoded payload.

Test plan

uv run pytest tests/ui/test_uuid_fastdatatable.py -v
# 13 passed

uv run pytest tests/ui -q
# 411 passed

Manual: against a MySQL table with a mediumblob column holding JSON, run SELECT, confirm the cell shows the JSON text, press v and confirm the preview appears without crashing, press V and confirm tree mode opens.