fix(fs): accept uploads without Content-Length

[CBOSSX]

Summary

• Fixes: PUT /api/fs/put returns a code 400 strconv.ParseInt error for an empty request whose Content-Length header is absent.
• Root cause: FsStream reparses the literal Gin header instead of using net/http's normalized Request.ContentLength, so an absent header becomes an empty string and fails before upload handling.
• A present malformed literal Content-Length is still rejected with code 400 before upload handling.

Closes #9606.

Regression evidence

• Before: go test ./server/handles -run '^TestFsStreamAcceptsMissingContentLength$' -count=1 -v exited 1

• After: go test ./server/handles -run '^TestFsStreamAcceptsMissingContentLength$' -count=1 -v exited 0

• Review regression: go test ./server/handles -run '^TestFsStreamRejectsInvalidContentLength$' -count=1 -v failed on the previous PR head and passes after the follow-up.

Verification

• go test -vet=off ./server/... -count=1
• go test -race ./server/handles -count=1
• go vet ./server/handles
• git diff --check

Validation notes

• go test ./server/... -count=1 reaches an unchanged Go 1.25 vet error in server/s3/backend.go; the same error reproduces on base 3e49fa4. Running the server suite with -vet=off passes, and go vet ./server/handles passes.
• go test -vet=off ./... -count=1 additionally requires local FUSE headers and a running aria2 service, and has an unrelated Windows-path assertion that fails on macOS. The affected server suite passes in full.

Scope

• 2 files changed, +54 / -5 lines

[skysliences]

请修改一下判断逻辑。增加对非法Content-Length的判断。逻辑可行。修改好重新提交pr

[CBOSSX]

已按建议补充非法 Content-Length 校验：缺失字面头时使用 Request.ContentLength，存在字面头时仍通过 strconv.ParseInt 校验，非法值在进入上传逻辑前返回 code 400。新增回归覆盖非法值；缺失/非法两个聚焦用例、server 全套（vet-off）、handles race、handles vet 和质量门禁均通过。