fix: resolve ~= specifiers with PEP 440 semantics (#1111)

## Summary

`~=` is the PEP 440 compatible release operator, but the resolver tries
`semver.validRange()` first, and semver accepts `~=` as a tilde range:

| specifier | semver | PEP 440 |
|---|---|---|
| `~=0.9` | `>=0.9.0 <0.10.0` | `>=0.9, ==0.*` |

So with the default `highest` strategy, `version: "~=0.9"` (or
`required-version = "~=0.9"` picked up through `version-file`) installs
the highest 0.9.x instead of the highest 0.x. The same happens with
`manifest-file`, where `tc.evaluateVersions` is semver-based.

This skips the semver evaluation for specifiers containing `~=`, so they
are only evaluated with `@renovatebot/pep440`. No semver range
legitimately uses `~=`.

Notes:

- Only a bare two-segment `~=X.Y` resolved incorrectly. `~=X.Y.Z` gives
the same result in both, and compound specifiers like `~=0.9, !=0.9.3`
were already rejected by semver and fell through to PEP 440.
- `minSatisfying` gets the same check for consistency. The lower bound
is the same under both interpretations, so `lowest` didn't produce wrong
results before.
- A single-segment `~=1` is invalid PEP 440. semver used to resolve it
to 1.x; it now fails with `No version found for ~=1`, same as other
invalid specifiers.

## Test plan

- Added tests for `~=0.9` with the default manifest and with
`manifest-file`. Both returned `0.9.26` instead of `0.10.2` before the
change.
- `npm run all` on Ubuntu (WSL) with Node 24.19.0: build, Biome (no
fixes), packaging, and 161 tests across 15 suites pass. The rebuilt
`dist/setup/index.cjs` only differs in the changed lines.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

src/version/resolve.ts

@@ -169,7 +169,9 @@ async function findHighestSatisfyingVersion(
 ): Promise<string | undefined> {
   // This fast path assumes uv releases are semver-monotonic: newer manifest
   // entries always have higher versions, with no lower-version backports.
-  const semverRange = semver.validRange(versionSpecifier);
+  const semverRange = isCompatibleReleaseSpecifier(versionSpecifier)
+    ? null
+    : semver.validRange(versionSpecifier);
   if (semverRange !== null) {
     const semverMatch = await getFirstMatchingVersion((version) =>
       semver.satisfies(version, semverRange),
@@ -193,11 +195,20 @@ async function findHighestSatisfyingVersion(
   return pep440Match;
 }
 
+// `~=` is the PEP 440 compatible release operator. semver accepts it as a
+// tilde range with a narrower upper bound (`~=0.5` as `<0.6.0` instead of
+// `<1.0`), so it must only be evaluated as PEP 440.
+function isCompatibleReleaseSpecifier(versionSpecifier: string): boolean {
+  return versionSpecifier.includes("~=");
+}
+
 function maxSatisfying(
   versions: string[],
   version: string,
 ): string | undefined {
-  const maxSemver = tc.evaluateVersions(versions, version);
+  const maxSemver = isCompatibleReleaseSpecifier(version)
+    ? ""
+    : tc.evaluateVersions(versions, version);
   if (maxSemver !== "") {
     core.debug(`Found a version that satisfies the semver range: ${maxSemver}`);
     return maxSemver;
@@ -218,7 +229,9 @@ function minSatisfying(
   versions: string[],
   version: string,
 ): string | undefined {
-  const minSemver = semver.minSatisfying(versions, version);
+  const minSemver = isCompatibleReleaseSpecifier(version)
+    ? null
+    : semver.minSatisfying(versions, version);
   if (minSemver !== null) {
     core.debug(`Found a version that satisfies the semver range: ${minSemver}`);
     return minSemver;