chore: pin LiteLLM to known good version (#1823)

Per internal guidance, pin LiteLLM to a known good version.

- Keeps the lower-bound specification (`>=1.74`)
- Effectively changes the upper-bound from (`<2.0` to `<1.82.7`)
- Added greppable string: `# pin to known-good version` for easier replacement.

Although `1.82.7` and `1.82.8` have already been purged from PyPI, still do this. We can revisit later when deemed appropriate. 

Closes

## By Submitting this PR I confirm:
- I am familiar with the [Contributing Guidelines](https://github.com/NVIDIA/NeMo-Agent-Toolkit/blob/develop/docs/source/resources/contributing/index.md).
- We require that all contributors "sign-off" on their commits. This certifies that the contribution is your original work, or you have rights to submit it under the same license, or a compatible license.
  - Any contribution which contains commits that are not Signed-Off will not be accepted.
- When the PR is ready for review, new or existing tests cover these changes.
- When the PR is ready for review, the documentation is up to date with these changes.




## Summary by CodeRabbit

* **Chores**
  * Updated litellm dependency version constraints to a specific range across multiple packages.

Authors:
  - Will Killian (https://github.com/willkill07)

Approvers:
  - https://github.com/mnajafian-nv
  - David Gardner (https://github.com/dagardner-nv)

URL: #1823

Author: willkill07

examples/frameworks/adk_demo/uv.lock

@@ -55,7 +55,7 @@ dependencies = [
   # Keep sorted!!!
   "nvidia-nat-core == {version}",
   "google-adk~=1.18",
-  "litellm~=1.74",
+  "litellm>=1.74,<1.82.7",  # pin to known-good version
 ]
 
 [tool.setuptools_dynamic_dependencies.optional-dependencies]

examples/frameworks/agno_personal_finance/uv.lock

@@ -56,7 +56,7 @@ dependencies = [
   "nvidia-nat-core == {version}",
   "agno>=1.2.3,<2.0.0",
   "google-search-results>=2.4.2,<3.0.0",
-  "litellm~=1.74",
+  "litellm>=1.74,<1.82.7",  # pin to known-good version
   "openai~=1.106",
 ]
 

packages/nvidia_nat_adk/pyproject.toml

@@ -57,7 +57,7 @@ dependencies = [
   # Keep sorted!!!
   "nvidia-nat-core == {version}",
   "crewai>=0.193.2,<1.0.0",
-  "litellm~=1.74",
+  "litellm>=1.74,<1.82.7",  # pin to known-good version
 ]
 
 [tool.setuptools_dynamic_dependencies.optional-dependencies]