release-please writes SemVer (not PEP 440) to pyproject.toml on prerelease bumps

[bokelley]

What happened

Release-please's Python release-type writes the raw SemVer version string to pyproject.toml's [project] version field, with no PEP 440 normalization for prerelease versions.

Observed on #817 (the v6.0.0-beta.1 release PR):

```
-version = "5.7.0"
+version = "6.0.0-beta.1" ← invalid PEP 440; PyPI rejects
```

Expected: version = "6.0.0b1" (PEP 440 form).

Worked around by hand-editing the release PR in commit 3e0f7e17 before merging, but the next prerelease bump will reproduce the same bug and need the same hand-edit. Need a permanent fix.

Background

The aao-ipr-bot flagged this on #804 (review comment). At the time we believed dropping the generic JSON extra-files updater would let the Python release-type's native pyproject.toml updater handle the conversion. That turned out to be wrong — the native updater also writes the raw SemVer string.

Options for a permanent fix

1. Custom updater via extra-files. Use an extra-files entry with type: "generic" or type: "python" (if such a type exists in release-please's schema) that knows PEP 440. Worth checking release-please's docs for a Python-aware pyproject.toml updater.
2. Post-bump GitHub Action. Add a step to the release-please workflow that runs after release-please opens/updates the release PR and normalizes the version. Cheap to write but adds a moving part.
3. Upstream fix in release-please. File against googleapis/release-please — this looks like a real bug in the Python strategy. Risky from a timeline perspective.
4. Switch off the prerelease line. Cut 6.0.0 GA without intermediate betas (won't work — we explicitly need beta testing on salesagent before GA).

I'd lean toward (2) as the immediate path and file (3) in parallel.

Acceptance

• Next prerelease bump (6.0.0-beta.2, etc.) produces version = "6.0.0b2" in pyproject.toml automatically, no hand-edit required.
• Stable version bumps still work (e.g., a future 6.0.0 GA cut produces version = "6.0.0").

Refs

• Surfaced on: chore(main): release 6.0.0-beta.1 #817 (release PR for v6.0.0-beta.1)
• Config history: release: cut 6.0.0-beta.1 line for AdCP 3.1 features #802 (initial 6.0-beta config), chore: configure release-please for 6.0.0-beta line (closes #802) #804 (extra-files removal that didn't fix it)

[bokelley]

Confirmed this reproduced again on release PR #834: release-please wrote pyproject.toml as 6.1.0-beta.1. I manually patched #834 to 6.1.0b1 in commit 00eae9d so this release can proceed. Keeping this issue open for the permanent automation fix so beta.2 does not need another hand edit.