fix(engine): use type-appropriate zero values for optional input defaults

[PolyphonyRequiem]

Problem

Optional workflow inputs without an explicit default: value previously defaulted to Python None. This causes two issues:

1. Templates render "None" — {{ workflow.input.optional_msg }} renders as the literal string "None" instead of empty string
2. | default() doesn't catch None — {{ workflow.input.optional_msg | default("fallback") }} still renders "None" because Jinja's default filter only catches undefined, not None (unless boolean=true is passed)

Reproduction

workflow:
  name: test-optional
  entry_point: echo
  input:
    required_id:
      type: number
      required: true
    optional_msg:
      type: string
      required: false
      # no default: specified
agents:
  - name: echo
    type: script
    command: pwsh
    args:
      - "-Command"
      - "Write-Output '{{ workflow.input.optional_msg | default(\"fallback\") }}'; exit 0"
    routes:
      - to: $end

conductor run test.yaml --input required_id=42

Expected: Script outputs fallback (or empty string)
Actual: Script outputs None

Fix

_apply_input_defaults() now uses type-appropriate zero values instead of None for optional inputs with no declared default:

Type	Zero value
string	""
number	0
boolean	false
array	[]
object	{}

Explicit default: values in the schema are still honored. This is a backward-compatible change — templates that checked if workflow.input.X will still work since zero values are falsy.

Changes

• src/conductor/engine/workflow.py: Add _TYPE_ZERO_VALUES class constant, update _apply_input_defaults() to use it
• tests/test_engine/test_workflow.py: Test verifying optional inputs render cleanly (no "None")

Testing

• 75 workflow engine tests pass (including new test)

[codecov-commenter]

Codecov Report

❌ Patch coverage is 75.00000% with 2 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (main@dbcf20f). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
src/conductor/engine/workflow.py	75.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #123   +/-   ##
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  Coverage        ?   84.84%           
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  Files           ?       53           
  Lines           ?     7172           
  Branches        ?        0           
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  Hits            ?     6085           
  Misses          ?     1087           
  Partials        ?        0           

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