XMover: Improve watermark percentage parsing with edge case handling

[coderabbitai]

Description

The watermark parsing logic in _parse_watermark_percentage method within cratedb_toolkit/admin/xmover/analysis/shard.py needs improvement to handle edge cases properly.

Problem

Currently, the function doesn't handle:

• Negative percentage values
• Percentage values greater than 100%
• Invalid input that could cause parsing errors

This could lead to incorrect watermark calculations and potential issues in shard analysis.

Proposed Solution

Add validation to:

1. Clamp values to valid range (0.0-100.0%)
2. Log warnings for invalid values
3. Return sensible defaults (85.0%) for invalid input
4. Handle ValueError exceptions from float conversion

Context

This improvement was identified during code review of PR #523.

References:

• PR: Admin/XMover: Add CrateDB shard analyzer and movement tool #523
• Comment: Admin/XMover: Add CrateDB shard analyzer and movement tool #523 (comment)
• Requested by: @amotl

Files to modify

• cratedb_toolkit/admin/xmover/analysis/shard.py (lines ~682-700)