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>

__tests__/download/download-version.test.ts

@@ -146,6 +146,28 @@ describe("download-version", () => {
       expect(mockGetAllVersions).not.toHaveBeenCalled();
     });
 
+    it("resolves ~= with PEP 440 compatible release semantics", async () => {
+      mockGetFirstMatchingVersion.mockImplementation(
+        (predicate: (version: string) => boolean) =>
+          ["1.0.0", "0.10.2", "0.9.26"].find(predicate),
+      );
+
+      const version = await resolveVersion("~=0.9", undefined);
+
+      expect(version).toBe("0.10.2");
+    });
+
+    it("resolves ~= with PEP 440 compatible release semantics from manifest-file", async () => {
+      mockGetAllVersions.mockResolvedValue(["1.0.0", "0.10.2", "0.9.26"]);
+
+      const version = await resolveVersion(
+        "~=0.9",
+        "https://example.com/custom.ndjson",
+      );
+
+      expect(version).toBe("0.10.2");
+    });
+
     it("treats == exact pins as explicit versions", async () => {
       const version = await resolveVersion("==0.9.26", undefined);