fix: correct unicode representation for jsonb_to_string

[sunng87]

I hereby agree to the terms of the GreptimeDB CLA.

Refer to a related PR or issue link (optional)

Fixes #7808

What's changed and what's your intention?

Correct unicode representation for json string.

PR Checklist

Please convert it to a draft if some of the following conditions are not met.

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.
• This PR requires documentation updates.
• API changes are backward compatible.
• Schema or data changes are backward compatible.

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses an issue where JSON strings generated from the internal JSONB representation might contain non-standard Rust-style Unicode escape sequences. The changes ensure that all Unicode characters are correctly represented using standard JSON escape formats, improving compatibility and preventing deserialization errors in downstream processes. The primary impact is a more robust and compliant JSON output from jsonb_to_string and related functions.

Highlights

• Unicode Representation Correction: The jsonb_to_string function now correctly handles Rust-style Unicode escape sequences (\u{...}) by converting them into standard JSON-compatible \uXXXX or surrogate pairs, ensuring proper JSON string output.
• Function Signature Change: The jsonb_string_to_serde_value function's return type has been updated from Result<serde_json::Value> to Result<String>, indicating that it now normalizes the JSON string rather than directly deserializing it.
• JSON Deserialization Flow Update: The jsonb_to_serde_json function now first normalizes the JSON string using the updated jsonb_string_to_serde_value and then explicitly parses the normalized string into a serde_json::Value.
• Test Case Refinement: Existing test cases were adjusted to reflect the new behavior, asserting against the normalized string output with standard JSON Unicode escapes, and new test categories were introduced for different error scenarios.

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[gemini-code-assist]

Code Review

This pull request correctly addresses an issue with Unicode representation when converting jsonb to a string by normalizing Rust-style Unicode escapes into standard JSON format. The logic is sound and the tests have been updated to reflect the new behavior. I have identified a couple of areas for improvement: a redundant function call that affects performance, and a misleading comment. My review includes specific suggestions to resolve these points.

[gemini-code-assist]

The jsonb_to_string function, called on the preceding line, is modified in this pull request to return an already-normalized JSON string. Consequently, this call to jsonb_string_to_serde_value is redundant and introduces unnecessary overhead by parsing and re-serializing an already valid string. You can simplify this by directly using the output of jsonb_to_string.

Suggested change

	let normalized = jsonb_string_to_serde_value(&json_string)?;
	serde_json::Value::from_str(&normalized).context(DeserializeSnafu { json: normalized })
	serde_json::Value::from_str(&json_string).context(DeserializeSnafu { json: json_string })

[avifenesh]

Same as metrics discussion — the LLM agent can inject the token value directly. Pre-check handles the missing token case.

[gemini-code-assist]

The comment states that "the original string is returned as-is" if it's valid JSON. However, the implementation at line 434 parses the string and then re-serializes it using v.to_string(). This process can alter the original string (e.g., by removing whitespace), so it's more of a canonicalization. The comment should be updated to reflect this behavior accurately.

Suggested change

	/// the original string is returned as-is.
	/// a canonicalized version of the string is returned.