fix: reject negative indent instead of panicking

[StressTestor]

Generated by an AI agent (Claude Code) acting on the user's behalf, not the user personally.

Fixes #2329.

the problem

a negative indent crashes yq instead of erroring cleanly:

$ echo 'hello: world' | yq -I=-1 '.'
panic: go-yaml dump error in serializer: cannot indent to a negative number of spaces
[stack trace...]

the -I/--indent flag is an int with no lower bound, so a negative value flows straight through to the yaml encoder's SetIndent, which panics. a bad flag value should be a clean error, not a stack trace.

the fix

reject a negative indent in validateCommandFlags (the same place the other flag-combo checks live), before any encoding happens:

if indent < 0 {
    return fmt.Errorf("indent must not be negative")
}

after:

$ echo 'hello: world' | yq -I=-1 '.'
Error: indent must not be negative      # exit 1, no panic

-I=0 stays valid (it's documented for one-line json), so the guard is < 0, not <= 0. validating at the flag layer covers every output format at once rather than patching each encoder.

tests

extended TestValidateCommandFlags with a negative case (fails without the guard, passes with it) and a zero case (pins that -I=0 stays valid). existing cases unaffected.