fix(ffmpeg/vulkan): close CBS context in Vulkan encoder teardown

[neatnoise]

Description

FFmpeg's Vulkan H.264/H.265/AV1 encoders never call ff_cbs_close(&enc->cbs) in their close functions, leaking CBS internal state (~1 MB H.264, ~8 MB H.265) per encoder session. This causes unbounded memory growth in applications that repeatedly create/destroy encoder contexts.

Fix adds ff_cbs_fragment_free() and ff_cbs_close() to all three Vulkan encoder close functions.

Verified with heaptrack:

• Before: 44 MB leaked
• After: 811 KB (normal overhead)

Issues Fixed or Closed

• Addresses Vulkan memory leak in fix(linux/pipewire): avoid memory leaks Sunshine#5360

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
• 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

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]

Can you submit this patch to FFmpeg as well?

[neatnoise]

I can't create PR's in FFmpeg project, created the issue https://code.ffmpeg.org/FFmpeg/FFmpeg/issues/23794