[1.9] optimize readPacket (#1705)

[methane]

Description

Avoid unnecessary allocation.

Although this is not a bug, this is a regression in 1.9 branch.
This PR fixes performance regression when compression is not used.

Checklist

• Code compiles correctly
• Created tests which fail without the change (if possible)
• All tests passing
• Extended the README / documentation, if necessary
• Added myself / the copyright holder to the AUTHORS file

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[Copilot]

Pull Request Overview

This PR optimizes the readPacket implementation to reduce unnecessary allocations and improve performance when compression is not used. The changes refactor the read logic by introducing a new readNext method on mysqlConn, update the compress functions to remove redundant reader function parameters, and add a convenient len method to the buffer type.

• Introduce mysqlConn.readNext to centralize buffer reading and timeout handling.
• Refactor compress.go to eliminate the extra readerFunc parameter.
• Add a len method to buffer.go to expose the buffer’s current length.

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
packets.go	Added a new readNext function and updated readPacket to use it.
compress.go	Removed the readerFunc parameter from readNext and readCompressedPacket.
buffer.go	Added a len method and updated readNext to work without error returns.

[coveralls]

coverage: 82.809% (-0.08%) from 82.884%
when pulling b32b0e8 on methane:opt-read-19
into 2ed589b on go-sql-driver:1.9.