feat(sdk/go): add audio and file multimodal helpers

[Sadit007]

Summary

Implements WithAudioFile, WithAudioURL, and WithFile multimodal helper functions for the Go SDK to achieve feature parity with the Python SDK. These functions enable Go agents to work with audio-capable and document-processing AI models from OpenAI, Anthropic, and Gemini without manually constructing content parts. All three functions properly base64-encode content, attach media type metadata, and produce valid provider-specific JSON compatible with LiteLLM.

Type of change

• New feature

Test plan

• cd sdk/go && go test ./... — all Go SDK tests pass, including multimodal
• TestWithAudioFile — tests local audio file reading with base64 encoding (mp3 format)
• TestWithAudioURL — tests HTTP download and base64 encoding with mock server (wav format)
• TestWithFile — parameterized test for generic file types: PDF, Word (.docx), CSV, JSON, TXT, HTML
• JSON serialization validated in each test to confirm provider compatibility
• Error handling: file not found, HTTP errors, network failures

Test coverage

• Ran tests for the surface I changed locally: cd sdk/go && go test ./... ✅
• New code paths are fully covered by unit tests:
  • WithAudioFile: reads file, base64 encodes, creates input_audio content part
  • WithAudioURL: downloads file via HTTP, base64 encodes, creates input_audio content part
  • WithFile: reads file, base64 encodes with data URL format, creates file content part
  • All tests include JSON serialization validation

Checklist

• I have linked any related issues (Fixes [Go SDK] Multimodal helpers missing: audio and generic file inputs not implemented #440)

Related issues / PRs

Fixes #440

[CLAassistant]

All committers have signed the CLA.

[AbirAbbas]

Thanks for the PR @Sadit007, if you could sign the CLA, I can get started on reviewing this!

[Sadit007]

Thanks for the PR @Sadit007, if you could sign the CLA, I can get started on reviewing this!

@AbirAbbas Thanks for the heads up! I've just signed the CLA. Let me know if you need anything else from me during the review process.

[github-actions]

Performance

SDK	Memory	Δ	Latency	Δ	Tests	Status
Go	217 B	-23%	0.58 µs	-42%	✓	✓

✓ No regressions detected

[github-actions]

📊 Coverage gate

Thresholds from .coverage-gate.toml: per-surface ≥ 86%, aggregate ≥ 88%, max per-surface regression ≤ 1.0 pp, max aggregate regression ≤ 0.50 pp.

Surface	Current	Baseline	Δ
control-plane	87.40%	87.30%	↑ +0.10 pp	🟡
sdk-go	91.80%	90.70%	↑ +1.10 pp	🟢
sdk-python	93.66%	93.63%	↑ +0.03 pp	🟢
sdk-typescript	92.63%	92.56%	↑ +0.07 pp	🟢
web-ui	89.69%	90.01%	↓ -0.32 pp	🟡
aggregate	88.85%	89.01%	↓ -0.16 pp	🟡

✅ Gate passed

No surface regressed past the allowed threshold and the aggregate stayed above the floor.

[github-actions]

📐 Patch coverage gate

Threshold: 80% on lines this PR touches vs origin/main (from .coverage-gate.toml:thresholds.min_patch).

Surface	Touched lines	Patch coverage	Status
control-plane	0	—	➖ no changes
sdk-go	83	96.00%	✅
sdk-python	0	—	➖ no changes
sdk-typescript	0	—	➖ no changes
web-ui	0	—	➖ no changes

✅ Patch gate passed

Every surface whose lines were touched by this PR has patch coverage at or above the threshold.

[AbirAbbas]

Reviewed locally — looks good. Pushed three small follow-ups directly to the branch:

• bound WithAudioURL with a 30s client timeout and a 50 MiB io.LimitReader cap so a slow or oversized URL can't hang the caller or exhaust memory
• dropped the empty Test_detectMIMEType stub (detectMIMEType is already covered by TestDetectMIMEType in multimodal_additional_test.go)
• added error-path tests for WithAudioFile / WithAudioURL / WithFile (read error, fetch error, HTTP non-200, size cap, file read error) so patch coverage stays above the gate — local coverage is now 100% on WithAudioFile/WithFile and 94.7% on WithAudioURL

Nice contribution, thanks for the parity work.

[Sadit007]

Thanks for the review and the final polish! Glad to contribute.