template: add base64encode and base64decode template functions

[pujitha24]

Motivation:
Notification templates have no way to base64-encode a string. Issue #3127
needs this to build a link into Karma, which expects its silence-search
query parameter to be a base64-encoded JSON payload
(?m=<base64(json)>), e.g. as an action button URL in a Slack message.

Approach:
Add base64encode and base64decode to the DefaultFuncs map in
template/template.go, alongside the existing toJson/reReplaceAll/
urlUnescape helpers. Both use base64.URLEncoding (the URL/filename-safe
alphabet) rather than StdEncoding, since the output is meant to be
embedded directly in a URL query parameter: StdEncoding's + and /
characters can be misinterpreted by URL/form parsers (+ is commonly
decoded as a space), silently corrupting the payload, which URLEncoding
avoids. The existing dict/list/toJson functions can already build
the JSON structure Karma expects; base64encode is the missing piece to
turn it into a link.

This only adds new template functions; no existing behavior changes.

Validation:
go build ./template/...
go test ./template/...
both pass, including new test cases in template/template_test.go for
base64encode, base64encode's URL-safe alphabet (verified against an
input whose encoding differs between Std and URL alphabets), base64decode,
and the base64decode error path for invalid input.

docs/notifications.md's function reference table is updated with both
new entries.

Fixes #3127

[ENHANCEMENT] Templates: Add `base64encode` and `base64decode` template functions

Signed-off-by: Pujitha Paladugu 10557236+pujitha24@users.noreply.github.com

[coderabbitai]

📝 Walkthrough

Walkthrough

The template function map now provides URL-safe base64encode and base64decode helpers. Documentation describes both functions, and template expansion tests cover successful encoding and decoding plus invalid input errors.

Changes

Base64 template helpers

Layer / File(s)	Summary
Template helpers and validation template/template.go, template/template_test.go, docs/notifications.md	DefaultFuncs registers URL-safe base64 encoding and decoding, decoding errors propagate through template execution, documentation lists both helpers, and tests cover success, URL-safe output, and invalid input.

Estimated code review effort: 2 (Simple) | ~10 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title is concise, specific, and matches the main change: adding base64 template helpers.
Description check	✅ Passed	The description covers motivation, approach, validation, docs, release notes, and the linked issue.
Linked Issues check	✅ Passed	The PR adds the requested base64 template support for Karma silence payloads and includes tests and docs, satisfying #3127.
Out of Scope Changes check	✅ Passed	The changes stay focused on the new template helpers, related tests, and docs, with no unrelated edits.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@template/template.go`:
- Around line 355-358: Update the base64decode helper to wrap DecodeString
errors with the helper name using fmt.Errorf and %w, returning the wrapped error
while preserving the empty decoded string and existing successful-decoding
behavior.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro Plus

Run ID: 23e4d3d5-0ab3-4c3b-a714-62bf66718eed

📥 Commits

Reviewing files that changed from the base of the PR and between 846bdbf and 8f5c910.

📒 Files selected for processing (3)

• docs/notifications.md
• template/template.go
• template/template_test.go