Fix collections sorting fallback

[stefangutica]

Reasoning

• When sorting collections by verifiedAndHolderCount, api_isVerified and api_holderCount are not present on every document in the tokens index.
• Without an explicit fallback, a missing value was interpreted as a very large negative number, which broke the searchAfter cursor and returned an internal server error.
• Elasticsearch sorts booleans as 0/1, so a missing api_isVerified should simply be treated as false, i.e. 0.

Proposed Changes

• Add missing: 0 to the api_isVerified sort field, so collections without the flag sort as not verified.
• Add missing: 0 to the api_holderCount sort field, so collections without a holder count sort as zero holders.

How to test

• GET /collections?sort=verifiedAndHolderCount&size=25 should return 200.
• Paginate further with the searchAfter cursor returned by the last item and confirm it works.

[github-actions]

k6 load testing comparison.
Base Commit Hash: b02a3e7
Target Commit Hash: 3f13262

Metric	Base				Target				Diff
	Avg	Max	90	95	Avg	Max	90	95	Avg	Max	90	95
Tokens	21.43	275.45	29.33	34.91	338.86	44984.58	111.87	121.50	+1481.45% 🔴	+16231.35% 🔴	+281.46% 🔴	+248.06% 🔴
Accounts	85.97	317.36	190.12	202.16	114.75	427.09	197.81	209.41	+33.47% 🔴	+34.58% 🔴	+4.05% 🔴	+3.59% 🔴
Transactions	52.16	10458.27	61.62	70.26	116.61	28510.14	104.05	112.40	+123.55% 🔴	+172.61% 🔴	+68.87% 🔴	+59.98% 🔴
Nodes	44.12	9068.52	39.70	75.16	189.78	15084.44	107.65	117.54	+330.12% 🔴	+66.34% 🔴	+171.15% 🔴	+56.37% 🔴
Mex	21.40	150.43	29.33	34.83	23.20	400.80	47.09	61.23	+8.41% 🔴	+166.43% 🔴	+60.55% 🔴	+75.82% 🔴
Pool	21.41	260.04	29.34	34.86	23.15	400.65	47.18	60.84	+8.11% 🔴	+54.07% 🔴	+60.77% 🔴	+74.53% 🔴
Blocks	30.92	517.61	51.33	58.68	43.16	432.59	93.87	104.91	+39.56% 🔴	-16.43% ✅	+82.89% 🔴	+78.77% 🔴
Test Run Duration	60001.25				60004.38

Legend: Avg - Average Response Time, Max - Maximum Response Time, 90 - 90th Percentile, 95 - 95th Percentile
All times are in milliseconds.