Harden get_pad_code schema, truncation, and environments

[adamtheturtle]

Summary

• Document max_file_chars as a UTF-8 byte budget with a default of 200,000 and keep truncation markers inside that budget.
• Mark malformed pad_environment_ids, missing/null nonbinary contents, and non-boolean binary flags as incomplete schema errors.
• Keep binary files when the text budget is exhausted; sanitize paths/titles; verify environment response IDs and pad ownership; bound concurrent environment fetches.

Fixes #120
Fixes #121
Fixes #122
Fixes #123
Fixes #124
Fixes #125
Fixes #126
Fixes #127
Fixes #128
Fixes #129
Fixes #130
Fixes #131
Fixes #132
Fixes #133
Fixes #134
Fixes #175
Fixes #176
Fixes #177
Fixes #178
Fixes #179
Fixes #180

Test plan

• swift test --filter PadCodeTests
• CI green

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Note

Medium Risk
Touches untrusted remote pad/environment payloads: path sanitization, ownership checks, and output size bounds. Logic is defensive and well-tested, but mistakes here could leak files across pads or blow MCP context.

Overview
Hardens get_pad_code so malformed API data cannot silently produce unbounded, incomplete, or cross-pad file lists.

Truncation and budgets: max_file_chars is documented as a UTF-8 byte cap (default 200k). The truncation marker now counts toward that budget. Binary files are kept after the text budget is exhausted.

Schema completeness: Malformed pad_environment_ids, missing/null nonbinary contents, non-boolean binary flags, missing file_contents, and unsafe paths/languages mark the payload incomplete with schema_errors. Environment responses must match the requested id and pad (pad_id).

Sanitization and fetch: Paths reject trailing slashes and empty components; flattened/suffixed names stay within path byte limits. Display fields strip Unicode format controls. Language→extension maps cover creatable pad languages. Environment fetches parse and drop each concurrent batch before the next so raw bodies are not retained all at once.

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