[enhancement]: bump revision to 2.0.4-SNAPSHOT after the 2.0.3 release

## Background

AgentScope Java `2.0.3` has been released to Maven Central (all modules, including `agentscope-bom`, now resolve to `2.0.3`. However, the `main` branch still carries:

- root `pom.xml`: `<revision>2.0.3-SNAPSHOT</revision>`
- `agentscope-dependencies-bom/pom.xml`: `<revision>2.0.3-SNAPSHOT</revision>`
- `agentscope-distribution/agentscope-bom/pom.xml`: `<revision>2.0.3-SNAPSHOT</revision>`

## Problem

Keeping `2.0.3-SNAPSHOT` on the development branch after `2.0.3` has shipped creates ambiguity:

1. **Version collision.** `2.0.3` (release) and `2.0.3-SNAPSHOT` (dev) coexist but point to different code states. Locally-installed SNAPSHOT artifacts are easy to confuse with the released ones when debugging.
2. **No development coordinate.** Work towards the next release has no reserved version; the bump would have to happen under time pressure during the next release train.
3. **Broken convention.** Standard Maven practice is to move `<revision>` to the next patch version immediately after a release, so `main` always represents "the next release under development".

## Proposal

Bump `<revision>` from `2.0.3-SNAPSHOT` to `2.0.4-SNAPSHOT` in all three places that define it:

- `pom.xml`
- `agentscope-dependencies-bom/pom.xml` (third-party dependency BOM)
- `agentscope-distribution/agentscope-bom/pom.xml` (the released `io.agentscope:agentscope-bom`, which manages AgentScope's own modules)

This is a three-file, three-line change thanks to the existing `${revision}` + `flatten-maven-plugin` setup — no per-module versions to touch.

## Optional follow-ups

- Document (or wire up) a one-command bump via `versions-maven-plugin` so future bumps are trivial:

  ```bash
  mvn org.codehaus.mojo:versions-maven-plugin:2.18.0:set-property \
      -Dproperty=revision -DnewVersion=2.0.5-SNAPSHOT -DgenerateBackupPoms=false
  ```

- (Further out) Automate this bump in the release workflow, so `main` never lags behind a published release.

## Impact

- Purely development-version housekeeping; no published artifacts are affected.
- Anyone building against `2.0.3-SNAPSHOT` should switch to the released `2.0.3`.

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