fix(cmd): reject NaN and Inf for numeric sizing flags

[cristim]

Summary

Rejects NaN and +/-Inf for --coverage, --target-coverage, --min-pool-size and --min-savings-pct in the flag validators (one requireFinite(name, v) helper), before any API call. Explicit --target-coverage 0 stays valid (documented disabled); ranges are unchanged.

Root cause: every ordered comparison with NaN is false, so --target-coverage NaN passed the <0 || >100 check, then applySizing/fetchExistingCoverage read TargetCoverage > 0 as "unset" and silently sized with --coverage (default 80). --coverage NaN reaches recfilter.ApplyCoverage (sizing.go:23), producing NaN HourlyCommitment/costs for SP recs and an arch-dependent int(NaN*count) for RI recs. --min-pool-size NaN passed the whole-number check; --min-savings-pct had no validation.

Evidence (offline)

• TestValidateNumericFlagsRejectNonFinite drives the real flags via rootCmd.ParseFlags then the validators; it never calls runTool, so no AWS client is built. Cases: NaN, nan, +/-Inf per flag, 1e999 (rejected by the flag parser), explicit 0 and valid values.

• Pre-fix (validators.go stashed): all 9 non-finite cases fail; post-fix pass. go vet ./..., build and go test ./cmd (463s) pass; pre-commit hooks (incl. gocyclo) pass.

Not in this PR

• Range bounds for --min-savings-pct: fix(cmd): --min-savings-pct has no range validation #2161.
• Stale "assume zero existing coverage" comments and CSV/dry-run effects of go#306: deferred to the pin-bump PR (the comments are accurate at pin 73d3366).

Closes #2136

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Reviewing files that changed from the base of the PR and between 22126ab and fa55e28.

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📒 Files selected for processing (3)

• CHANGELOG.md
• cmd/validators.go
• cmd/validators_test.go

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