fix: correct append_struct reserve calculation and optimize insertion

[chaowyc]

Description

This patch fixes two issues in append_struct():

1. Fixed reserve() calculation: Changed from reserve(data_len) to

   reserve(buf.size() + data_len) to correctly account for existing

   buffer size. The original code could fail to allocate sufficient

   space when the buffer already contains data.

2. Optimized insertion: Replaced loop with N push_back() calls with

   a single insert() operation, reducing function call overhead and

   avoiding potential mid-loop reallocations.

Screenshot

N/A

Issues Fixed or Closed

N/A - This addresses a bug identified through code analysis.

Roadmap Issues

N/A

Type of Change

• feat: New feature (non-breaking change which adds functionality)
• fix: Bug fix (non-breaking change which fixes an issue)
• docs: Documentation only changes
• style: Changes that do not affect the meaning of the code (white-space, formatting, missing semicolons, etc.)
• refactor: Code change that neither fixes a bug nor adds a feature
• perf: Code change that improves performance
• test: Adding missing tests or correcting existing tests
• build: Changes that affect the build system or external dependencies
• ci: Changes to CI configuration files and scripts
• chore: Other changes that don't modify src or test files
• revert: Reverts a previous commit
• BREAKING CHANGE: Introduces a breaking change (can be combined with any type above)

Checklist

• Code follows the style guidelines of this project
• Code has been self-reviewed
• Code has been commented, particularly in hard-to-understand areas
• Code docstring/documentation-blocks for new or existing methods/components have been added or updated
• Unit tests have been added or updated for any new or modified functionality

AI Usage

• None: No AI tools were used in creating this PR
• Light: AI provided minor assistance (formatting, simple suggestions)
• Moderate: AI helped with code generation or debugging specific parts
• Heavy: AI generated most or all of the code changes

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[ReenigneArcher]

This appears to be a duplicate of your other PR #4415

[chaowyc]

This appears to be a duplicate of your other PR #4415

Yes, that's correct — I previously pushed this PR to my master branch, so it appeared in my earlier PR (#4415), which was merged from master. Feel free to merge the latest one and close this one.