feat(s3): add short access links and SDK core

[xqvvu]

Summary

• Add short S3 download alias URLs (/api/system/file/d/<alias>.<exp>.<sig>) and short upload session URLs (/api/system/file/u/<token>).
• Extract the access-link protocol into @fastgpt-sdk/storage/access-link so other trusted services can reuse HMAC signing, TTL bucketing, alias reuse, upload token hashing, verification, and revoke logic.
• Keep FastGPT runtime concerns in service adapters: Mongo stores, route builders, proxy handlers, and error mapping.
• Preserve legacy JWT download/upload route compatibility for already-issued links while new signing paths issue short access links.
• Update dataset/search/quote preview URL generation to use async shortlink signing from the previous shortlink implementation.

Tests

• pnpm exec vitest run -c vitest.config.mts test/sdk/storage/accessLink.test.ts
• pnpm --filter @fastgpt-sdk/storage build
• pnpm --filter @fastgpt/app typecheck
• From packages/service: pnpm exec vitest run -c vitest.config.ts test/common/s3/accessLink.test.ts test/common/s3/token.test.ts test/core/dataset/utils.test.ts test/core/dataset/data/controller.test.ts
• From projects/app: pnpm exec vitest run -c vitest.config.ts test/api/system/file/accessLink.test.ts test/api/system/file/sourceContentType.test.ts
• git diff --check

Notes

• Replaces feat(s3): add short access links #7262.
• PR2 upload policy validation and PR3 ChatBox upload abort remain out of scope.

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Coverage Report

Status	Category	Percentage	Covered / Total
🔵	Lines	18.43%	1632 / 8855
🔵	Statements	18.41%	1713 / 9300
🔵	Functions	16.21%	342 / 2109
🔵	Branches	15.1%	759 / 5024

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