fix(purchase): tighten the stored-note reason slot and pin two slot limits #1827

Follow-up from the review of #1820 (#1721). None of these block, and all are small.

1. Invalid byte in the reason slot. quotedTextPattern (internal/purchase/errnotes.go) accepts a raw invalid byte in the operator's resolve reason. Go's regexp reads the byte as U+FFFD, which the class doesn't exclude, so reason="a\xffb" passes unchanged. The %q writer never produces this, and JSON output turns it into U+FFFD, so the effect is cosmetic. Fix: exclude \x{FFFD} from the class, with a test.
2. Allowlist comment overstates the rule. The comment (errnotes.go, around lines 15-17) says no slot matches whitespace. The reason slot accepts an ASCII space on purpose, so reword the comment.
3. Two probes survive. Add one assertion each:
   • Removing the {1,256} length limit on commitmentIDPattern breaks no test. Assert that a 257-character commitment ID falls back to the category message.
   • Widening the reason escape from \\[ -~] to \\. breaks no test. Assert that a backslash followed by a raw U+202E in the reason falls back.

Also noted, and older than #1820: skipped-rec notes whose Cost Explorer platform value contains spaces (for example "Red Hat Enterprise Linux") fall back to the category message, because the identity slot excludes spaces. The pattern before #1820 failed on these too. Widen it only if the fallback hides something users need.