refactor: inject http.Client into CheckExternalLinks to enable httptest.NewTestServer

[shinagawa-web]

Overview

CheckExternalLinks constructs its own *http.Client internally, making it impossible to inject a test client from outside.
Go 1.27 introduced httptest.NewTestServer, whose Client() routes requests to the in-process server transparently — so tests can use real production URLs instead of replacing them with the test server address.
To take advantage of this, the function must accept a client from the caller.

Problem

// internal/rule/external_link.go
client := &http.Client{
    Timeout: time.Duration(timeoutSeconds) * time.Second,
}

Because client is created inside CheckExternalLinks, tests are forced to rewrite every URL in the test markdown to point to the temporary localhost server:

markdown := fmt.Sprintf(`[ok](%s/ok)`, ts.URL)  // real URL is lost

This means tests cannot verify that production code actually requests the correct host or path.

Proposed change

Add a *http.Client parameter to CheckExternalLinks.
When nil is passed, the function falls back to its current behaviour (constructs a default client using timeoutSeconds).

func CheckExternalLinks(
    path string, ctx *preprocess.Context, offset int,
    skipPatterns []*regexp.Regexp,
    timeoutSeconds, retryDelayMs, maxConcurrency, maxRetries int,
    allowedStatuses []int, urlCache *sync.Map,
    perHostConcurrency, perHostIntervalMs int,
    client *http.Client,   // nil → build default
) ([]LintError, int)

Effect on tests (Go 1.27+)

ts := httptest.NewTestServer(handler)
// ts.Client() routes any host to the in-process server
results, _ := rule.CheckExternalLinks(
    "mock.md", ctx, 0,
    nil, 10, 10, rule.DefaultMaxConcurrency, rule.DefaultMaxRetries,
    nil, &sync.Map{}, 0, 0,
    ts.Client(),   // injected
)

Markdown in tests can now contain literal production-like URLs (https://example.com/ok) while still being served by the test server — letting tests assert on method, host, and path correctness.

Effect on the linter (production)

// internal/linter/linter.go — pass nil; behaviour unchanged
rule.CheckExternalLinks(..., nil)

References

• Go 1.27 release notes: net/http/httptest.NewTestServer
• https://zenn.dev/catatsuy/articles/8e8dbf6afd1196

[shinagawa-web]

Implementation Plan

Files to modify

• internal/rule/external_link.go — add client *http.Client parameter; build default client when nil
• internal/linter/linter.go — pass nil at the call site (linter.go:412)
• internal/rule/external_link_test.go — pass nil for now; switch to ts.Client() after Go 1.27 bump
• internal/rule/external_link_network_test.go — same as above

Checklist

• Update CheckExternalLinks signature and nil-guard
• Update linter.go call site
• Update all test call sites to pass nil (no behaviour change)
• Bump go.mod to Go 1.27
• Rewrite tests to use httptest.NewTestServer + ts.Client()
• make test-all