Create MySQL_Session.md

doc/internal/MySQL_Session.md

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+### Flowchart of `MySQL_Session::RunQuery()`
+
+This function mostly calls `MySQL_Connection::async_query()` with the right arguments.  
+Returns an integer status code indicating the result of the query execution:
+- 0: Query execution completed successfully.
+- -1: Query execution failed.
+- 1: Query execution in progress.
+- 2: Processing a multi-statement query, control needs to be transferred to MySQL_Session.
+- 3: In the middle of processing a multi-statement query.
+
+```mermaid
+---
+title: MySQL_Session::RunQuery()
+---
+flowchart TD
+RQ["MySQL_Connection::async_query()"]
+BEGIN --> RQ
+RQ --> END
+```
+
+### Flowchart of `MySQL_Session::handler()`
+
+WORK IN PROGRESS
+
+```mermaid
+---
+title: MySQL_Session::handler()
+---
+flowchart TD
+RQ["rc = RunQuery()"]
+RC{rc}
+CBCS["rc1 = handler_ProcessingQueryError_CheckBackendConnectionStatus()"]
+RC1{rc1}
+RQ --> RC
+RC -- 0 --> OK
+RC -- -1 --> CBCS
+CBCS --> RC1
+CS["CONNECTING_SERVER"]
+ReturnMinus1["return -1"]
+RC1 -- -1 --> ReturnMinus1
+RC1 -- 1 --> CS
+HM1CLE1["handler_minus1_ClientLibraryError()"]
+HM1CLE2["handler_minus1_ClientLibraryError()"]
+myerr1{"myerr >= 2000
+&&
+myerr < 3000"}
+RC1 --> myerr1
+myerr1 -- yes --> HM1CLE1
+HM1CLE1 -- true --> CS
+HM1CLE1 -- false --> ReturnMinus1
+HM1LEDQ1["handler_minus1_LogErrorDuringQuery()"]
+myerr1 -- no --> HM1LEDQ1
+HM1HEC1["handler_minus1_HandleErrorCodes()"]
+HM1LEDQ1 --> HM1HEC1
+HM1HEC1 -- true --> HR1{"handler_ret"}
+HR1 -- 0 --> CS
+HR1 --> RHR1["return handler_ret"]
+HM1GEM1["handler_minus1_GenerateErrorMessage()"]
+HM1HEC1 -- false --> HM1GEM1
+RE["RequestEnd()"]
+HM1HBC1["handler_minus1_HandleBackendConnection()"]
+HM1GEM1 --> RE
+RE --> HM1HBC1
+```
+
+
+### Flowchart of `MySQL_Session::handler_ProcessingQueryError_CheckBackendConnectionStatus()`
+TODO