chore: configure release-please for 6.0.0-beta line (closes #802) (#804)

* chore: configure release-please for 6.0.0-beta line

Enable prerelease versioning so release-please bumps within the beta line
(6.0.0-beta.1 → 6.0.0-beta.2 → …) rather than jumping to a stable 6.0.0
on every breaking-change commit.

The next release-please run produces v6.0.0-beta.1; subsequent feature
work for the AdCP 3.1 SDK surface (#741 canonical-formats, #347
UpstreamRecorder, #711 permissive resolver, #758 revoked_publisher_domains
inline) ships as additional beta increments until GA.

Refs: #802

Release-As: 6.0.0-beta.1

* fix: drop extra-files pyproject.toml writer to preserve PEP 440 normalization

Per aao-ipr-bot review on #804: the generic JSON `extra-files` updater
writes the raw SemVer version string into `pyproject.toml`, bypassing
PEP 440 normalization. For stable releases this is inert (SemVer ==
PEP 440), but for prereleases it produces invalid versions like
`6.0.0-beta.1` instead of `6.0.0b1` — and PyPI rejects the wheel.

The Python release-type natively handles `pyproject.toml`'s
`[project] version` field and applies the SemVer → PEP 440 conversion
automatically. Dropping the redundant `extra-files` entry lets the
native updater do its job.

Refs: #802, #804

Author: bokelley

release-please-config.json

@@ -7,13 +7,8 @@
       "bump-minor-pre-major": true,
       "bump-patch-for-minor-pre-major": false,
       "versioning": "default",
-      "extra-files": [
-        {
-          "type": "json",
-          "path": "pyproject.toml",
-          "jsonpath": "$.project.version"
-        }
-      ]
+      "prerelease": true,
+      "prerelease-type": "beta"
     }
   },
   "changelog-sections": [