fix: allow configured file endpoints through agent proxy

[zyssyz123]

Summary

• resolve INTERNAL_FILES_URL with explicit precedence over SERVER_CONSOLE_API_URL and FILES_URL, including the common Docker case where the variable is present but empty
• pass the resolved file origin to agent_ssrf_proxy and generate a narrow allow rule for the configured host while preserving private-network denial for other destinations
• document the HTTPS CONNECT trust boundary and add coverage for configuration precedence, private file hosts, blocked non-file paths, metadata protection, and HTTPS tunneling

Fixes #39733

Screenshots

Not applicable; this is a backend and Docker proxy fix.

Validation

• 30 configuration unit tests passed
• 18 file-signing and file-request service tests passed
• Ruff format and lint passed
• Pyrefly passed
• Docker Compose generation and configuration validation passed
• Agent Squid integration suite passed locally and on Linux/amd64, including private target denial and HTTPS CONNECT coverage
• deployed revision c52c2da0ee3df64f35f86ea23fd7986cfb442369 to the dev environment
• dev upload request, upload, download request, and byte-for-byte download round trip passed against agent-upload.dify.dev
• API, workers, agent backend, and local sandbox remained healthy with zero restarts after deployment

Checklist

• I understand that this PR may be closed in case there was no previous discussion or issues.
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the Docker documentation accordingly.
• I ran the full repository-wide make lint && make type-check; focused backend lint and type checks passed.

From Codex

[github-actions]

Pyrefly Type Coverage

Metric	Base	PR	Delta
Type coverage	56.86%	56.86%	+0.00%
Strict coverage	56.38%	56.38%	+0.00%
Typed symbols	37,167	37,170	+3
Untyped symbols	28,439	28,441	+2
Modules	3092	3092	0

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.18%. Comparing base (676dff0) to head (47b98d8).

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #39839   +/-   ##
=======================================
  Coverage   86.18%   86.18%           
=======================================
  Files        5086     5086           
  Lines      287691   287697    +6     
  Branches    57356    57357    +1     
=======================================
+ Hits       247950   247958    +8     
+ Misses      34938    34936    -2     
  Partials     4803     4803           

Flag	Coverage Δ
api	86.37% <100.00%> (+<0.01%)	⬆️

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[laipz8200]

This function discards the scheme and port. For example, a user-configured https://files.internal:8443 would be parsed as files.internal, resulting in https://files.internal:443 being forwarded. Is this expected?

[laipz8200]

Please confirm if the attacker could exploit URL normalization in Nginx to bypass this pattern matching, such as using /files/../console/api/...