[repo-assist] Fix inconsistent ~> LOCKEDVERSION twiddle range for short version strings

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🤖 This is an automated response from Repo Assist, an AI assistant.

Closes #3727

Root cause

In paket.template, the bare LOCKEDVERSION/CURRENTVERSION placeholder (with no :Major/:Minor/:Patch/:Build spec) substituted the raw, un-normalized locked version text (e.g. "0.68") verbatim into the dependency line, instead of a normalized Major.Minor.Patch form.

That raw text then feeds into the ~> (twiddle) operator parser, which infers which segment to bump based on how many numeric fragments are present in the text (DependenciesFileParser.twiddle). As a result:

• Locked version 0.68 (2 segments) → ~> LOCKEDVERSION produced [0.68, 1.0.0)
• Locked version 0.68.1 (3 segments) → ~> LOCKEDVERSION produced [0.68.1, 0.69.0)

Both represent "the same" minor version conceptually, but yielded very different (and for the 2-segment case, unintended) upper bounds — exactly the inconsistency reported in #3727.

Fix

In TemplateFile.fs, when no explicit segment spec is given, the substituted version text is now padded to at least 3 segments (Major.Minor.Patch), preserving any pre-release/build suffix from the original source text. This makes ~> LOCKEDVERSION (and similar unspecced placeholders) behave consistently regardless of how many segments the locked version happens to have in paket.lock.

This only affects the no-spec case (segmentCount = 0); explicit :Major/:Minor/:Patch/:Build specs are unchanged.

Trade-offs

• This changes behavior for any existing paket.template relying on the old (inconsistent) raw substitution for short (1–2 segment) locked versions combined with ~>. However, the old behavior was a bug with no clear intentional use, so this is considered a bugfix rather than a breaking change.
• Exact-match (Specific) substitution behavior for bare LOCKEDVERSION (without ~>) was already normalizing 2-segment versions to 3 segments in existing tests (e.g. "1.2" → "1.2.0"), so this fix is consistent with, and does not change, that existing behavior.

Test Status

• dotnet test tests/Paket.Tests/Paket.Tests.fsproj --filter "FullyQualifiedName~TemplateFile" (net9.0): 104/104 passed (including 3 new regression test cases added for this fix).
• dotnet test tests/Paket.Tests/Paket.Tests.fsproj --filter "FullyQualifiedName~Version|FullyQualifiedName~Template|FullyQualifiedName~Twiddle" (net9.0): 294/294 passed.
• net461 test target could not run in this sandbox (missing Mono host) — this is an environment limitation, not a code issue; net9.0 results are the reliable signal here.

AI Disclosure

This PR was authored autonomously by Repo Assist, an automated AI assistant for this repository. Please review carefully before merging.

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