chore: eval extra's google-cloud-aiplatform bound has no upper cap, unlike all/gcp/test

## 🔴 Required Information

**Describe the Bug:**
Not a crash report — a version-constraint consistency question. `pyproject.toml`'s `eval` optional-dependency extra constrains `google-cloud-aiplatform` with no upper bound, while three sibling extras that also depend on `google-cloud-aiplatform` all cap it at `<2`. Quoted verbatim, at `origin/main` `c7ffcfa8`:

- `optional-dependencies.all` (pyproject.toml:82): `"google-cloud-aiplatform[agent-engines,evaluation]>=1.148.1,<2"`
- `optional-dependencies.eval` (pyproject.toml:195): `"google-cloud-aiplatform[evaluation]>=1.148"` — **no upper bound**
- `optional-dependencies.gcp` (pyproject.toml:225): `"google-cloud-aiplatform[agent-engines]>=1.148.1,<2"`
- `optional-dependencies.test` (pyproject.toml:290): `"google-cloud-aiplatform[agent-engines,evaluation]>=1.148.1,<2"`

Three of the four extras that pull in `google-cloud-aiplatform` cap it below the 2.x major line; `eval` — the one most users installing for evaluation support (`pip install google-adk[eval]`) actually reach — does not.

**Steps to Reproduce:**
1. Install `google-adk[eval]` in a fresh environment with no version overrides (e.g. `pip install -e ".[eval]"` against a checkout of this repo, or `pip install "google-adk[eval]"` once published).
2. Check the resolved `google-cloud-aiplatform` version (`pip show google-cloud-aiplatform`).
3. Compare against what `pip install "google-adk[all]"`, `[gcp]`, or `[test]` would resolve for the same package.

**Expected Behavior:**
All four extras that depend on `google-cloud-aiplatform` apply the same version bound, or the difference is intentional and documented.

**Observed Behavior:**
`pip install google-adk[eval]` resolves `google-cloud-aiplatform` on the 2.x line (observed: `2.1.0`), while `[all]`, `[gcp]`, and `[test]` all resolve to the last version under 2 (observed: `1.165.1`) for the identical base requirement. No error or crash is being reported here — this issue is scoped to the constraint inconsistency itself; whether the `eval` extra's own missing upper bound is intentional (e.g. evaluation-specific APIs needing 2.x) or an oversight from when the bound was added to the other three extras is what's being asked.

**Environment Details:**
- ADK Library Version (pip show google-adk): unreleased, `google/adk-python` `origin/main` `c7ffcfa8` (`__version__` reports `2.8.0`)
- Desktop OS: Windows 11
- Python Version (python -V): 3.13.5

**Model Information:**
- Are you using LiteLLM: N/A
- Which model is being used: N/A

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## 🟡 Optional Information

**Regression:**
N/A — this is the first time this specific constraint has been checked, not a regression from a previously-working state.

**Logs:**
N/A — no error output; this is a constraint-consistency observation, not a failure report.

**Additional Context:**
Happy to send a one-line PR adding `<2` to the `eval` extra's `google-cloud-aiplatform` bound if that's the intended fix, once confirmed.

**How often has this issue occurred?:**
- Always (100%) — the constraint as written is deterministic, not intermittent.