feat(api): Add filterContext field to Lua type in EnvoyExtensionPolicy

[norman-zon]

What type of PR is this?

feat/api

What this PR does / why we need it:

Adds an optional filterContext field (map[string]string) to the Lua type in EnvoyExtensionPolicySpec, exposing Envoy's existing LuaPerRoute.filter_context field.

This allows a shared Lua script (stored in a ConfigMap via ValueRef) to be parameterized differently per route using request_handle:filterContext(). Without this, users must either duplicate the entire script inline with hardcoded values for each route or use EnvoyPatchPolicy to manually patch the generated xDS — both of which are fragile and defeat the purpose of script reuse.

Example usage:

lua:
  - type: ValueRef
    valueRef:
      name: shared-auth-lua
      kind: ConfigMap
    filterContext:
      token_header: x-api-key

The Lua script accesses the values via:

local ctx = request_handle:filterContext()
local header_name = ctx["token_header"]

Changes:

• api/v1alpha1/lua_types.go — add FilterContext map[string]string to Lua struct
• internal/ir/xds.go — add FilterContext to IR Lua struct
• internal/gatewayapi/envoyextensionpolicy.go — pass FilterContext from API → IR
• internal/xds/translator/lua.go — set FilterContext on LuaPerRoute, converting map[string]string → structpb.Struct
• GatewayAPI and xDS translator testdata updated
• E2E test added: Lua script reads filterContext() and sets value as response header
• Documentation updated with FilterContext section in Lua task guide
• Release note added

No new Envoy features required — this only exposes an existing LuaPerRoute field.

Which issue(s) this PR fixes:

Fixes #8715

Release Notes: Yes

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.28%. Comparing base (faf7a99) to head (070ba24).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8730      +/-   ##
==========================================
- Coverage   74.32%   74.28%   -0.04%     
==========================================
  Files         245      245              
  Lines       38801    38807       +6     
==========================================
- Hits        28837    28829       -8     
- Misses       7962     7973      +11     
- Partials     2002     2005       +3     

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[zirain]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. What shall we delve into next?

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[zirain]

FAIL: TestE2E/LuaHTTP/http_route_with_lua_filter_context (60.01s) e2e failed.

[rudrakhp]

You will need to add mocks for the filterContext() api in the validator mocks which is failing your E2E. Please refer how other APIs are mocked for this.