fix(feishu): stop logging tenant access token fragments

[davien]

Description

GetTenantAccessToken logs the first 8 and last 4 characters of every freshly issued tenant_access_token at Info level (internal/datasource/connector/feishu/core/client.go, [Feishu] got tenant_access_token: %s...%s). Tenant access tokens are bearer credentials for the whole Feishu app; partial fragments in shared logs narrow brute-force space and are flagged by secret scanners. This PR keeps the refresh event and expiry but drops every token character:

logger.Infof(ctx, "[Feishu] tenant access token refreshed expire=%ds", result.Expire)

Type of Change

• 🐛 Bug fix

Testing

• Added TestTenantAccessTokenNeverAppearsInLogs (fails before the fix): a fake token endpoint returns a long and a 3-character token; the captured log must keep expire=7200s and contain neither the token nor its 8-character prefix / 4-character suffix, and the cached token must not trigger a second request.
• go test -race ./internal/datasource/connector/feishu/core/
• git diff --check origin/main...HEAD

Related Issue

None filed; the problem and reproduction are described above.

Checklist

• git diff --check origin/main...HEAD passes
• Changed source files are formatted (gofmt -l is clean)
• Targeted tests for the changed packages/components pass
• Diff-scoped lint passes where applicable — golangci-lint was not available in my environment; I will follow up on any CI lint findings
• Full-repository checks were run, or any unrelated/environment-dependent failures are documented above
• Self-reviewed the code
• Added/updated tests covering the change
• Updated related documentation — not needed, no user-facing configuration or API shape changes
• Breaking changes are clearly called out in the description above (none)

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