Add property-style tests for base64 padding and uuidv7 shifts

[NikolayS]

What

Adds property-style regression tests in src/test/regress that close input-boundary gaps around two recent fixes:

• 1f41403 "Fix truncation rules for base64 encoding" — the base64url work (e1d9171) accidentally relaxed = padding acceptance for plain base64; the fix re-gates the relaxed rule on the url variant.
• 7575215 "Reject infinite and out-of-range interval shifts in uuidv7()".

Why

The commits above each shipped a single targeted test. This adds broader property coverage so the restored boundaries stay locked in:

strings (base64 / base64url):

• Round-trip identity encode(decode(encode(x))) = x for both variants across input lengths 0..8, covering every padding phase (len % 3 in {0,1,2}). Uses a deterministic byte pattern, so output is stable.
• Padding/truncation rejection matrix: each unpadded trailing-group length (1/2/3 chars) × variant, asserting the exact error. base64 rejects every unpadded partial group (invalid base64 end sequence); base64url accepts 2- and 3-char remainders but rejects a lone 1-char remainder; misplaced = padding (unexpected "=" while decoding ...) is rejected in both.

uuid (uuidv7 interval):

• Monotone-ordering property: strictly increasing uuidv7(interval) shifts yield strictly increasing (sortable) UUID values, comparing the uuid values themselves rather than only the extracted timestamps. Complements the infinite-interval and out-of-range rejection tests from 7575215.

All uuidv7 tests assert properties/errors only — never literal random UUID values.

Test evidence

Built with meson setup build --buildtype=debug -Dcassert=true -Dtap_tests=enabled and ninja -C build.

ok  - test_setup
ok  - strings   (267 ms)
ok  - uuid      (128 ms)
# All 3 tests passed.

Full parallel_schedule run: strings (test 22) and uuid (test 16) both ok, regression.diffs empty.

Notes

Test-only change: 4 files, +188 lines (sql/strings.sql, sql/uuid.sql, and matching expected/*.out).

Closes #60

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