unescape: Decode \u escaped characters for surrogate pairs correctly

[cosmo0920]

Currently, we ignore surrogate pairs for \u escape on Unicode representation.
To handle this, we need to process with surrogate pairs manner.
Noe that this representation is also encoded \uXXXX representation on creating JSON.
On creating msgpack, this unescaping operation is effective.

Closes #9712.

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Testing
Before we can approve your change; please submit the following in a comment:

• Example configuration file for the change

$ bin/fluent-bit -i stdout -o stdout

and send {"text": "\ud83e\udd17"} in the same terminal.

• Debug log output from testing the change

Fluent Bit v4.0.0
* Copyright (C) 2015-2024 The Fluent Bit Authors
* Fluent Bit is a CNCF sub-project under the umbrella of Fluentd
* https://fluentbit.io

______ _                  _    ______ _ _             ___  _____ 
|  ___| |                | |   | ___ (_) |           /   ||  _  |
| |_  | |_   _  ___ _ __ | |_  | |_/ /_| |_  __   __/ /| || |/' |
|  _| | | | | |/ _ \ '_ \| __| | ___ \ | __| \ \ / / /_| ||  /| |
| |   | | |_| |  __/ | | | |_  | |_/ / | |_   \ V /\___  |\ |_/ /
\_|   |_|\__,_|\___|_| |_|\__| \____/|_|\__|   \_/     |_(_)___/ 


[2025/01/06 18:17:45] [ info] Configuration:
[2025/01/06 18:17:45] [ info]  flush time     | 1.000000 seconds
[2025/01/06 18:17:45] [ info]  grace          | 5 seconds
[2025/01/06 18:17:45] [ info]  daemon         | 0
[2025/01/06 18:17:45] [ info] ___________
[2025/01/06 18:17:45] [ info]  inputs:
[2025/01/06 18:17:45] [ info]      stdin
[2025/01/06 18:17:45] [ info] ___________
[2025/01/06 18:17:45] [ info]  filters:
[2025/01/06 18:17:45] [ info] ___________
[2025/01/06 18:17:45] [ info]  outputs:
[2025/01/06 18:17:45] [ info]      stdout.0
[2025/01/06 18:17:45] [ info] ___________
[2025/01/06 18:17:45] [ info]  collectors:
[2025/01/06 18:17:45] [ info] [fluent bit] version=4.0.0, commit=09214ebc7b, pid=74663
[2025/01/06 18:17:45] [debug] [engine] coroutine stack size: 36864 bytes (36.0K)
[2025/01/06 18:17:45] [ info] [storage] ver=1.2.0, type=memory, sync=normal, checksum=off, max_chunks_up=128
[2025/01/06 18:17:45] [ info] [simd    ] NEON
[2025/01/06 18:17:45] [ info] [cmetrics] version=0.9.9
[2025/01/06 18:17:45] [ info] [ctraces ] version=0.5.7
[2025/01/06 18:17:45] [ info] [input:stdin:stdin.0] initializing
[2025/01/06 18:17:45] [ info] [input:stdin:stdin.0] storage_strategy='memory' (memory only)
[2025/01/06 18:17:45] [debug] [stdin:stdin.0] created event channels: read=25 write=26
[2025/01/06 18:17:45] [debug] [input:stdin:stdin.0] buf_size=16000
[2025/01/06 18:17:45] [debug] [stdout:stdout.0] created event channels: read=28 write=29
[2025/01/06 18:17:45] [ info] [sp] stream processor started
[2025/01/06 18:17:45] [ info] [output:stdout:stdout.0] worker #0 started
{"text": "\ud83e\udd17"}
[2025/01/06 18:17:47] [debug] [task] created task=0x6000032ec000 id=0 OK
[2025/01/06 18:17:47] [debug] [output:stdout:stdout.0] task_id=0 assigned to thread #0
[0] stdin.0: [[1736155066.742995000, {}], {"text"=>"🤗"}]
[2025/01/06 18:17:47] [debug] [out flush] cb_destroy coro_id=0
[2025/01/06 18:17:47] [debug] [task] destroy task=0x6000032ec000 (task_id=0)
^C[2025/01/06 18:17:48] [engine] caught signal (SIGINT)
[2025/01/06 18:17:48] [ info] [output:stdout:stdout.0] thread worker #0 stopping...
[2025/01/06 18:17:48] [ info] [output:stdout:stdout.0] thread worker #0 stopped

• Attached Valgrind output that shows no leaks or memory corruption was found

==80122== 
==80122== HEAP SUMMARY:
==80122==     in use at exit: 0 bytes in 0 blocks
==80122==   total heap usage: 2,999 allocs, 2,999 frees, 1,341,761 bytes allocated
==80122== 
==80122== All heap blocks were freed -- no leaks are possible
==80122== 
==80122== For lists of detected and suppressed errors, rerun with: -s
==80122== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

If this is a change to packaging of containers or native binaries then please confirm it works for all targets.

• Run local packaging test showing all targets (including any new ones) build.
• Set ok-package-test label to test for all targets (requires maintainer to do).

Documentation

• Documentation required for this feature

Backporting

• Backport to latest stable release.

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Fluent Bit is licensed under Apache 2.0, by submitting this pull request I understand that this code will be released under the terms of that license.

[vit-zikmund]

Thanks for following up this quick @cosmo0920!
I see you're struggling with the error cases there. As the unescaping function is expected to return the number of processed bytes, wouldn't it be better to stick to that and in the error cases set the ch being returned to the replacement character (ch = L'\uFFFD') I suggested in my issue comment footnote?

On the other hand, staying strict and rejecting that sequence is likely much better for the user, who won't suddenly find magic replacements in their data.