Move the complexity gate from ESLint to Biome

[rodrigobnogueira]

Summary

This repo used ESLint for one rule, sonarjs/cognitive-complexity, with
@typescript-eslint/parser only to parse TypeScript, and that parser
refuses TypeScript 7, so a lint dependency gated the compiler. Biome
enforces the same ceiling of 15 with noExcessiveCognitiveComplexity
(biome.json), has no TypeScript dependency, and drops the ESLint
toolchain, as @nest-native/jobs did. The report reads Biome's JSON and
refuses an empty scope or a truncated run; CI's base-branch report
degrades to no baseline when the tooling changed since the base commit.

Scope Check

• This PR is package/library work.
• This PR is sample/docs/CI work.
• If this is sample work, it does not include packages/trpc/**
  changes.

Validation

• Relevant local checks are listed below.

# npm run complexity:check and complexity:report pass under Biome

Security Pass

• Dependency changes reviewed: ESLint, @typescript-eslint/parser and eslint-plugin-sonarjs removed; @biomejs/biome added (dev only).
• Runtime behavior reviewed: none; dev tooling only.
• Secrets or sensitive data checked: none involved.

[github-actions]

📊 Coverage Report

Metric		PR	Base	Diff
Statements	████████████████████	660/660 (100%)	660/660 (100%)	⚪ 0%
Branches	████████████████████	280/280 (100%)	280/280 (100%)	⚪ 0%
Functions	████████████████████	122/122 (100%)	122/122 (100%)	⚪ 0%
Lines	████████████████████	635/635 (100%)	635/635 (100%)	⚪ 0%

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[github-actions]

⏱️ Performance Report

✅ Tests	216 passed, 0 failed, 0 skipped
🧪 Suites	43
⏱️ Total step time	10.00s (install + tests) 🟢 -1.00s
⚙️ Test execution	3.13s 🟢 -81ms

🐢 Slowest test suites

#	Suite	Tests	Duration	vs Base
1	test/generators/schema-generator.spec.ts	23	2.26s	🟢 -67ms
2	test/adapter/trpc-client-adapter-e2e.spec.ts	16	185ms	⚪ ~0
3	test/adapter/trpc-http-adapter.spec.ts	19	75ms	⚪ ~0
4	test/router/trpc-router.spec.ts	29	38ms	⚪ ~0
5	test/context/trpc-context-creator.spec.ts	26	36ms	⚪ ~0
6	test/adapter/trpc-http-adapter-unit.spec.ts	22	20ms	⚪ ~0
7	test/generators/zod-serializer.spec.ts	38	11ms	⚪ ~0
8	test/router/trpc-router-lifecycle.spec.ts	11	10ms	⚪ ~0
9	test/decorators/decorators.spec.ts	11	7ms	⚪ ~0
10	test/module/trpc-module.spec.ts	9	7ms	⚪ ~0

🐌 Slowest individual tests

#	Suite	Test	Duration	vs Base
1	schema-generator	generateSchemaContent (type-level AppRouter contract) should require a link transformer on typed clients when generated with hasTransformer	1.41s	🟢 -60ms
2	schema-generator	generateSchemaContent (type-level AppRouter contract) should typecheck generated AppRouter with a typed tRPC client	845ms	⚪ ~0
3	trpc-client-adapter-e2e	real @trpc/client adapter E2E express handles typed queries, mutations, and context extraction	70ms	⚪ ~0
4	trpc-client-adapter-e2e	real @trpc/client adapter E2E express streams subscriptions through the real client	22ms	⚪ ~0
5	trpc-client-adapter-e2e	real @trpc/client adapter E2E express runs class-validator DTO validation through ValidationPipe	21ms	⚪ ~0
6	trpc-client-adapter-e2e	real @trpc/client adapter E2E fastify handles typed queries, mutations, and context extraction	12ms	⚪ ~0
7	trpc-client-adapter-e2e	real @trpc/client adapter E2E fastify runs class-validator DTO validation through ValidationPipe	10ms	⚪ ~0
8	trpc-http-adapter	TrpcHttpAdapter edge cases should handle Fastify GET with createContext	10ms	⚪ ~0
9	trpc-client-adapter-e2e	real @trpc/client adapter E2E express reports Zod input errors through the real client	8ms	⚪ ~0
10	trpc-client-adapter-e2e	real @trpc/client adapter E2E fastify streams subscriptions through the real client	6ms	⚪ ~0
11	trpc-client-adapter-e2e	real @trpc/client with superjson transformer E2E express round-trips Date instances through queries	6ms	⚪ ~0
12	trpc-http-adapter	TrpcHttpAdapter (server config passthrough) Express should apply responseMeta headers before streaming SSE responses	6ms	⚪ ~0
13	trpc-client-adapter-e2e	real @trpc/client adapter E2E fastify reports Zod input errors through the real client	5ms	⚪ ~0
14	trpc-client-adapter-e2e	real @trpc/client with superjson transformer E2E express round-trips Date instances through mutation inputs and outputs	5ms	⚪ ~0
15	trpc-http-adapter	TrpcHttpAdapter Express handler should apply method pipes and interceptors through HTTP requests	5ms	⚪ ~0

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[github-actions]

🧠 Cognitive Complexity Report

Metric	PR	Base	Diff
Total complexity	201	159	🔴 +42
Max function complexity	15	12	🔴 +3
Functions measured	43	53	⚪ -10

🧩 Most complex functions

Function	Location	Complexity	Status
serializeZodSchema	packages/trpc/generators/zod-serializer.ts:9	15	🟡 Watch
TrpcContextCreator.createHandler → callback	packages/trpc/context/trpc-context-creator.ts:255	13	🟢 OK
generateSchemaContent	packages/trpc/generators/schema-generator.ts:59	11	🟢 OK
TrpcHttpAdapter.resolveBody	packages/trpc/trpc-http-adapter.ts:124	9	🟢 OK
TrpcContextCreator.extractHttpExceptionMessage	packages/trpc/context/trpc-context-creator.ts:479	7	🟢 OK
createProcedureDecorator → callback	packages/trpc/decorators/procedure.decorator.ts:35	7	🟢 OK
TrpcRouter.createSubscriptionProcedure → callback	packages/trpc/trpc-router.ts:320	7	🟢 OK
TrpcRouter.resolveContextId	packages/trpc/trpc-router.ts:504	7	🟢 OK
TrpcRouter.validateSubscriptionOutput	packages/trpc/trpc-router.ts:551	7	🟢 OK
mapProcedure	packages/trpc/generators/schema-generator.ts:104	6	🟢 OK

🧭 Cognitive complexity is reported as a review signal, not a merge gate. Prefer small, intention-revealing refactors when complexity rises.

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