fix(#907): default new agents to PYTHON_3_13 to avoid CloudFormation rejection

[aidandaly24]

Description

Fixes the bug reported in #907: the CLI defaulted new agents to PYTHON_3_14,
which CloudFormation rejects with AWS::EarlyValidation::PropertyValidation
in regions other than us-west-2 / us-east-1. The resulting Change Set
fails immediately and leaves the stack stuck in REVIEW_IN_PROGRESS,
requiring manual stack deletion to recover.

Root cause

DEFAULT_PYTHON_VERSION in src/schema/constants.ts was set to PYTHON_3_14,
so any new project created via agentcore add agent (and any consumer that
falls back to the default — e.g. schema-mapper.ts, lib/packaging/python.ts,
lib/packaging/index.ts) baked an unsupported runtime into agentcore.json.

Fix (minimal)

• Change DEFAULT_PYTHON_VERSION from PYTHON_3_14 → PYTHON_3_13.
• Keep PYTHON_3_14 in PythonRuntimeSchema so users in supported regions
  (us-west-2 / us-east-1) can still opt in explicitly.
• Add JSDoc on both PythonRuntimeSchema and DEFAULT_PYTHON_VERSION that
  references issue PYTHON_3_14 runtime accepted by CLI but rejected by CloudFormation #907 and documents why the default deliberately lags the
  newest enum entry.

Tests

• New DEFAULT_PYTHON_VERSION (issue #907) describe block in
  src/schema/__tests__/constants.test.ts:
  • asserts the default is PYTHON_3_13 (and not PYTHON_3_14)
  • asserts the default is a valid member of PythonRuntimeSchema
  • asserts PYTHON_3_14 is still accepted as an explicit opt-in
  • regression invariant: the default must NOT equal the newest entry in
    PythonRuntimeSchema — prevents future lockstep bumps from re-opening PYTHON_3_14 runtime accepted by CLI but rejected by CloudFormation #907
• Updated src/cli/operations/agent/generate/__tests__/schema-mapper.test.ts
  to assert against DEFAULT_PYTHON_VERSION (also pinning the literal
  'PYTHON_3_13' for clarity), exercising the consumer path that produces
  the agentcore.json runtime field.

A companion PR in aws/agentcore-l3-cdk-constructs
on branch fix/907-a15ec7fe applies the same default change to
DEFAULT_MCP_PYTHON_VERSION.

Related Issue

Closes #907

Documentation PR

N/A — no user-facing documentation change required. The behavior change is a
default-value adjustment with explanatory JSDoc added in source.

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update
• Other (please describe):

Testing

How have you tested the change?

• I ran npm run test:unit and npm run test:integ
• I ran npm run typecheck
• I ran npm run lint
• If I modified src/assets/, I ran npm run test:update-snapshots and committed the updated snapshots

Targeted runs:

npx vitest run --project unit \
  src/cli/operations/agent/generate/__tests__/schema-mapper.test.ts \
  src/schema/__tests__/constants.test.ts
# Test Files  2 passed (2)
#      Tests  80 passed (80)

npm run typecheck runs clean.

Checklist

• I have read the CONTRIBUTING document
• I have added any necessary tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly
• I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• My changes generate no new warnings
• Any dependent changes have been merged and published

---

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

[agentcore-cli-automation]

Approving — the fix is minimal, targeted, and well-justified.

Verified locally:

• DEFAULT_PYTHON_VERSION is the single source of truth for the default Python runtime across the CLI: it's re-exported from src/cli/tui/screens/{agent,generate,mcp}/*, used in AddAgentScreen.tsx (3 call sites), GatewayTargetPrimitive.ts (MCP creation), schema-mapper.ts, and both packaging paths (lib/packaging/python.ts, lib/packaging/index.ts). Flipping the constant fixes all of these in one shot.
• PYTHON_3_14 remains a valid enum member of PythonRuntimeSchema, so users in us-west-2 / us-east-1 who set it explicitly still work. The new JSDoc explains this well.
• The existing tests that exercise PYTHON_3_14 (e.g. lib/packaging/__tests__/python.test.ts, schema/schemas/__tests__/agent-env.test.ts) only verify enum membership or version-string extraction, so they're unaffected by the default change.
• The AGENTS.md / snapshot documentation enumerates all valid runtimes (not the default), so no snapshot update is needed.

No issues worth blocking on.

Out of scope for this PR (just flagging for tracking, not requesting changes):

• DEFAULT_MCP_PYTHON_VERSION in aws/agentcore-l3-cdk-constructs is still PYTHON_3_14. That only matters for consumers who use the CDK constructs directly (without the CLI); the CLI's MCP path in GatewayTargetPrimitive.ts passes DEFAULT_PYTHON_VERSION explicitly, so CLI users are fine. The companion PR mentioned in the description should still land to close the gap for direct-CDK users.
• PYTHON_VERSION_OPTIONS in src/cli/tui/screens/mcp/types.ts still labels PYTHON_3_14 as 'Latest' and lists it first — but it's currently unused, so harmless.

[github-actions]

Coverage Report

Status	Category	Percentage	Covered / Total
🔵	Lines	43.19%	9034 / 20915
🔵	Statements	42.47%	9592 / 22585
🔵	Functions	40.02%	1558 / 3893
🔵	Branches	40%	5811 / 14527

Generated in workflow #2648 for commit 7d2c584 by the Vitest Coverage Report Action