feat: enforce mypy type checking across all source files

[Hweinstock]

Issue #, if available: #167

Description of changes:

Problem

There is no static analysis run to determine type mismatches to catch "obvious" bugs/errors. Additionally, the SDK has a mypy config and a py.typed marker but mypy is never run — not in CI, not in pre-commit. There are 211 type errors across 21 source files.

mypy appears to be a standard already used by other teams and projects. Ex. strands-agents already enforces mypy.

Solution

• Add type annotations and add uv run mypy src/ to the CI lint job.
• Avoid any changes that could affect runtime logic — every change is either a type annotation or a # type: ignore[error-code] suppression.

Significant # type: ignore directives remain for issues that can't be fixed without logic changes (boto3 returns Any, strands type mismatches, TypedDict method limitations). These can be fixed as a follow-up, but carry risks for code that is not covered by integration tests.

Testing

• ran mypy locally via uv run mypy src/.
• lint/format CI job ran this against PR.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 99.39024% with 1 line in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (main@11e2ac5). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
src/bedrock_agentcore/memory/client.py	92.30%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #335   +/-   ##
=======================================
  Coverage        ?   90.85%           
=======================================
  Files           ?       42           
  Lines           ?     4048           
  Branches        ?      624           
=======================================
  Hits            ?     3678           
  Misses          ?      203           
  Partials        ?      167           

Flag	Coverage Δ
unittests	90.85% <99.39%> (?)

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