Add upper-bound chunk-size validation to put

[AndreyVMarkelov]

Summary

• Rejects --chunksize values below 4MiB or above 128MiB with distinct error messages, in addition to the existing multiple-of-4MiB check.
• Extracts putChunkSizeUnit and putMaxChunkSize constants to replace inline magic numbers.
• Updates flag descriptions for --chunksize and --workers to clarify they apply to chunked large-file uploads.
• Adds a README section explaining upload sessions and when to tune chunk size/workers.
• Converts the chunk-size test to a table-driven test covering all three validation branches.

Test plan

• go test ./cmd/ -run TestPutChunkSize passes all three cases
• go build ./... succeeds
• go run ./tools/gen-docs output matches docs/commands/dbxcli_put.md
• CI passes