fix: Instrumentation and templatespec

[regularkevvv]

Summary

• Fix TemplateSpec.from_file() path stripping bug when using a pre-configured environment
• Add comment-based markers ({#jt:anchor:name#}, {#jt:trace:name#}) as an alternative to function call syntax

Bug Fix: from_file Path Stripping

When TemplateSpec.from_file() was called with a pre-configured environment, the directory portion of the path was incorrectly stripped, causing template loading to fail for nested paths.

Before (broken):

env = Environment(loader=FileSystemLoader("prompts/"))
spec = TemplateSpec.from_file("resume_v2/sections/system.prompt", env=env)
# Error: TemplateNotFound: system.prompt
# Tried to load "prompts/system.prompt" instead of "prompts/resume_v2/sections/system.prompt"

After (fixed):

env = Environment(loader=FileSystemLoader("prompts/"))
spec = TemplateSpec.from_file("resume_v2/sections/system.prompt", env=env)
# Works correctly - loads "prompts/resume_v2/sections/system.prompt"

Changes

┌─────────────────────────────────────────┬───────────────────────────────────┬─────────────────────────────────┐
│                Scenario                 │              Before               │              After              │
├─────────────────────────────────────────┼───────────────────────────────────┼─────────────────────────────────┤
│ from_file("a/b/c.j2", env=my_env)       │ Loads "c.j2" (fails)              │ Loads "a/b/c.j2" (works)        │
├─────────────────────────────────────────┼───────────────────────────────────┼─────────────────────────────────┤
│ from_file("a/b/c.j2")                   │ Creates env at a/b/, loads "c.j2" │ Same (unchanged)                │
├─────────────────────────────────────────┼───────────────────────────────────┼─────────────────────────────────┤
│ from_file("c.j2", env=instrumented_env) │ Re-instruments env                │ Reuses existing instrumentation │
└─────────────────────────────────────────┴───────────────────────────────────┴─────────────────────────────────┘

New Feature: Comment-Based Markers

Added support for comment-based markers that allow jinjatest to be a dev-only dependency:

{#jt:anchor:system#}
System prompt content here.

{#jt:anchor:user#}
User: {{ user_input }}
{#jt:trace:user_rendered#}

These are automatically transformed to {{ jt.anchor("system") }} calls when test_mode=True.

Test Plan

• Added 8 new tests for from_file with provided environment
• Added tests for template_dir parameter behavior
• Added tests for comment markers with provided env
• All 487 tests passing