feat(tools): NuGet enrichment in get_dependency_blast_radius

[manus-use]

Summary

Adds .NET / NuGet enrichment to get_dependency_blast_radius, completing six-ecosystem coverage for the blast-radius tool:

Ecosystem	Enricher	Status
npm	_enrich_npm	✅ merged
PyPI	_enrich_pypi	✅ merged
Maven	_enrich_maven	✅ merged
crates.io (Rust)	_enrich_crates	🟡 PR #103 open
RubyGems (Ruby)	_enrich_rubygems	🟡 PR #104 open
NuGet (.NET)	_enrich_nuget	✅ this PR

What changed

src/manus_agent/tools/get_dependency_blast_radius.py

New helpers:

• _extract_nuget_first_release(pages) — walks NuGet registration pages to find the earliest published date and count total versions; handles all ISO-8601 variants (Z suffix, +HH:MM offset, no-TZ naive, sub-second fractions including 7-digit .NET ticks); isolated per-item try/except so one malformed date doesn't abort the scan.

• _enrich_nuget(name) — two-call enrichment, consistent with the other enrichers:

  1. NuGet Search API (azuresearch-usnc.nuget.org/query?q={name}&prerelease=false) → total_downloads, latest_version, description (truncated to 120 chars), home_page
  2. NuGet Registration API (api.nuget.org/v3/registration5/{name}/index.json) → total_versions, first_release_date, age_years

  Both calls are individually guarded; failure of either does not prevent the other from returning data.

Updated:

• _enrich_package dispatch — routes "nuget" / ".net" / "dotnet" / "csharp" / "c#" to _enrich_nuget.
• _blast_score — when weekly_downloads is absent, proxies from total_downloads / max(age_years * 52, 1). NuGet's search API returns all-time totals rather than per-week stats; the same fallback strategy is used by the crates.io and RubyGems enrichers in PRs feat: add crates.io enrichment to get_dependency_blast_radius #103/feat(tools): RubyGems enrichment in get_dependency_blast_radius #104.
• NuGet output block added to the main tool report (latest version, total versions, all-time downloads, first-release date with age, description, home page).
• Module-level docstring updated (added sources 7 & 8; NuGet added to per-ecosystem metrics list).

tests/test_nuget_enrichment.py (new, 34 tests)

Class	Coverage
TestExtractNugetFirstRelease	All ISO-8601 variants, multi-page aggregation, malformed date skipping, empty pages, age_years, total_versions
TestEnrichNuget	Happy path, case-insensitive ID match, description truncation, search error, registration error, both errors, non-matching result, version count
TestEnrichPackageNuGetDispatch	All five ecosystem aliases routing to _enrich_nuget
TestBlastScoreNuGetFallback	total_downloads proxy, weekly_downloads precedence, zero-age guard

Suite result: 1192 passed, 0 failures (was 1158 before this PR).

No real HTTP calls

All 34 tests use unittest.mock.patch on requests.get. Zero network I/O.

Existing open PRs checked (no overlap)

Confirmed no duplication against: #51, #53, #54, #58, #60, #64, #65, #67, #74, #75, #76, #77, #78, #79, #80, #82, #83, #85, #86, #87, #88, #89, #90, #96, #97, #98, #100, #103, #104

None of the open PRs implement NuGet enrichment or touch _enrich_nuget.