Add serpingapi as a web search provider

[musabgultekin]

Following up on our email thread.

Adds serpingapi as an optional third choice for --search-provider, next to exa and google. Opt-in only: exa stays the default and nothing changes for existing users.

What it adds

• src/search/search.go: serpingapiSearch / parseSerpingapiResponse alongside googleSearch (POST https://api.serpingapi.com/v1/search, X-API-Key header, organic[] title/link/snippet mapped into the existing SearchResult), plus PerformSerpingapiSearchWithParams, which goes through the same is-fast content extraction and formatResultsForAI path as the Google provider. The extraction loop is factored into extractContentForResults so both providers share it.
• Site filters from query optimization are sent as a site: operator in the query, since the API has no separate parameter.
• main.go, src/helper/helper.go, src/structs/structs.go, md/usage.md: flag help text and provider validation.
• SEARCH_SETUP.md: a short configuration section.
• No new dependencies.

How to configure

export SERPINGAPI_API_KEY="your_key"
tgpt --search-provider serpingapi -f "your question"   # or SEARCH_PROVIDER=serpingapi

A free plan exists (1,000 searches/month, no card). API reference: https://serpingapi.com/docs

How it was tested

• go build ./... and go vet ./... clean; gofmt clean on the touched Go files.
• go test ./...: everything passes except src/helper TestDetectPackageManager (Windows-path subtests), which fails identically on unmodified main when run on macOS.
• New unit tests in src/search/search_test.go cover the response mapping (invalid links skipped, empty and malformed JSON) and the site-filter query.
• A live request with an invalid key returns the documented 401 invalid_api_key through the existing error path. Happy to paste a full tgpt -f run with a real key if useful.

[gitar-bot]

Code Review ✅ Approved

Adds serpingapi as an optional third web search provider alongside exa and google. The implementation includes response parsing, content extraction shared with the Google provider, site filter handling, and comprehensive unit tests. No issues found.

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[aandrew-me]

Great!