harden JsonEscaper by removing sprintf and clarifying byte handling

[rootvector2]

Additional Description:

• Replaces C-style sprintf writes in JSON escaping with explicit fixed-position hex encoding for control bytes.
• Uses unsigned-byte comparison for control character detection to avoid signed char ambiguity on non-ASCII input.
• Preserves existing escaping behavior while reducing error-prone string write patterns.
• Adds regression coverage for high-bit bytes (0x80, 0xFF) and mixed high-bit/control-byte input.
• AI usage disclosure: GitHub Copilot assistance was used to draft this change and description; the author manually reviewed the code and tests.

Risk Level:
Low. The change is localized to JSON escaping utility logic and test coverage was expanded for edge cases.

Testing:

• Added JsonEscapeTest coverage for high-bit byte pass-through.
• Added JsonEscapeTest coverage for mixed high-bit and control-byte escaping.
• Attempted targeted test: bazel test --config=clang //test/common/common:logger_test
• Local run was blocked by environment setup (missing JDK and CC toolchain resolution). Full validation expected in CI.

Docs Changes:
None.

Release Notes:
None.

Platform Specific Features:
None.

[Optional Runtime guard:]
Not needed.

[Optional Fixes #Issue]
N/A

[Optional Fixes commit #PR or SHA]
N/A

[Optional Deprecated:]
N/A

[repokitteh-read-only]

Hi @rootvector2, welcome and thank you for your contribution.

We will try to review your Pull Request as quickly as possible.

In the meantime, please take a look at the contribution guidelines if you have not done so already.

🐱

Caused by: #44474 was opened by rootvector2.

see: more, trace.

[phlax]

@rootvector2 please fix DCO

[agrawroh]

@rootvector2 Please fix the failing triggers.

/wait