cmd: add --data flag to opa inspect for data file inspection

[madmecodes]

Summary

• Adds --data flag to opa inspect to allow inspecting individual JSON and YAML data files
• Mirrors the existing single .rego file inspection support (PR Add ability for opa inspect to inspect a single file outside of any bundle #6873) for data files
• The flag can be repeated to inspect multiple data files

Fixes #6879

Changes

• cmd/inspect.go: Added dataPaths field to params, wired up --data flag, updated validation to support either positional args or --data (not both), updated command help text
• internal/bundle/inspect/inspect.go: Added DataFileInfo() function that validates and inspects JSON/YAML data files, mapping them to the data namespace
• cmd/inspect_test.go: Added tests for JSON inspection, YAML inspection, invalid JSON handling, non-data file rejection, and mutual exclusivity of --data with positional args

Usage

# Inspect a single JSON data file
opa inspect --data data.json

# Inspect a YAML data file
opa inspect --data config.yaml

# Inspect multiple data files
opa inspect --data data.json --data config.yaml

Test plan

• All 81 existing inspect tests pass
• All 4 existing inspect package tests pass
• 5 new tests added and passing
• CI validation