[repo-assist] Distinguish "package not found" from real conflicts in resolver errors

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Closes #3446

Root cause

When Paket's resolver fails with only a single unmet requirement (no other requirement constraining the same package), it previously always printed:

Conflict detected:
  - Falanx.Tool 0.4.0-alpha5 requested package Falanx.Proto.Codec.Json: >= 0.4.0-alpha5 (alpha)
  - Available versions:
    - 0.3.0

As reported in #3446, this is misleading: there's no actual conflict between multiple requirements here — the requested package/version range simply wasn't available from any configured source. "Conflict" troubleshooting (multiple requirements pulling in different directions) is a very different mental model from "the version you asked for doesn't exist".

Fix

In PackageResolver.fs, buildConflictReport now takes a header string parameter. The single-requirement error path (cfs.Count = 1) now uses the header "Package not found" instead of "Conflict detected". The multi-requirement path (Count > 1, a genuine conflict between contradictory requirements) keeps "Conflict detected" unchanged.

Trade-offs

• This only changes the header text of the error message; the rest of the report (available versions list, requesting package info) is unchanged, minimizing risk of breaking anything relying on the surrounding format.
• Existing integration test updateShouldFindPackageConflict (used by ResolverSkipsConflictsFastSpecs.fs) checks for "Conflict detected" combined with multiple requirements from Ninject's scenario, which produces a genuine multi-requirement conflict, so it is unaffected by this change (I verified there are no other tests asserting on the exact "Conflict detected" string for a single-requirement case).

Test Status

• dotnet build src/Paket.Core/Paket.Core.fsproj -c Debug: succeeded (net461 + netstandard2.0), pre-existing warnings only, no new ones introduced.
• dotnet test tests/Paket.Tests/Paket.Tests.fsproj --filter "FullyQualifiedName~Resolver": 33 passed, 0 failed, 5 skipped (net9 target). The net461 target could not run in this sandbox because Mono is not installed (pre-existing environment limitation, unrelated to this change).
• Integration tests were not run in this sandbox due to the same lack of Mono for net461; the affected code path only changes a string header, and I manually confirmed by code review that the one integration test referencing "Conflict detected" text targets a genuine multi-requirement conflict scenario, so it remains unaffected.

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